Add saving and loading the configuration to a file (discussion #610)

Revives #640, closed only to clear a review backlog. Two entries in
the Configuration menu: "Enregistrer la configuration sous..." saves
the settings to a .conf file, "Charger une configuration..." replaces
them with one. The file is always ini format, so a profile made on one
platform loads on the others.

Changes from #640:
- Window sizes, positions and dock layouts (".../geometry",
  ".../state") and the recent-files lists stay out of a profile and
  are kept on load: they describe this computer, not a preference.
- A saved file carries a format marker; loading refuses files that
  QElectroTech did not save.
- Loading closes QElectroTech and asks the user to start it again,
  instead of starting a second copy: a second copy started while this
  one is still running hands its arguments over and exits (main.cpp).
- The colour dialog's custom colours are reloaded from the profile
  before quitting, since ~QETApp() saves them over the loaded ones.
- Both actions are registered with ShortcutManager, so they appear in
  command search and can be given a shortcut.
- The copy rules live in the header-only utils/configprofile.h and
  are unit tested (tst_configprofile).

Loading still replaces rather than merges: a setting the profile does
not have is removed, or switching profiles would keep settings from the
previous one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/utils/configprofile.h

@@ -0,0 +1,98 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef CONFIGPROFILE_H
+#define CONFIGPROFILE_H
+
+#include <QSettings>
+#include <QString>
+#include <QStringList>
+
+/**
+	Configuration profiles (discussion #610): the settings of QElectroTech
+	saved to a file, and loaded back from one.
+
+	Two kinds of keys stay out of a profile, because they describe this
+	computer rather than how the user wants QElectroTech to behave: window
+	sizes, positions and dock layouts (".../geometry", ".../state"), and
+	the recent-files lists ("...-recentfiles/..."). They are not written to
+	the file, and loading a profile keeps the current ones.
+*/
+namespace ConfigProfile
+{
+		/// Key written to every exported file, so that loading refuses a
+		/// file that was not exported by QElectroTech. Never copied into
+		/// the live settings.
+	inline const QString marker_key{QStringLiteral("qelectrotech-configuration/format")};
+
+		/// True for a key that stays out of a profile (see above).
+	inline bool isLocalKey(const QString &key)
+	{
+		return key.endsWith(QLatin1String("/geometry"))
+			|| key.endsWith(QLatin1String("/state"))
+			|| key.contains(QLatin1String("-recentfiles/"))
+			|| key == marker_key;
+	}
+
+		/// True if @a file was exported by exportTo().
+	inline bool isProfile(const QSettings &file)
+	{
+		return file.value(marker_key).toInt() == 1;
+	}
+
+		/// Replace the contents of @a file with every key of @a live,
+		/// except the local ones. Returns the number of keys written.
+	inline int exportTo(const QSettings &live, QSettings &file)
+	{
+		file.clear();
+		int written = 0;
+		const QStringList keys = live.allKeys();
+		for (const QString &key : keys) {
+			if (isLocalKey(key)) {
+				continue;
+			}
+			file.setValue(key, live.value(key));
+			++written;
+		}
+		file.setValue(marker_key, 1);
+		file.sync();
+		return written;
+	}
+
+		/// Replace every key of @a live, except the local ones, with the
+		/// keys of @a file. A key that is not in @a file is removed rather
+		/// than kept: loading a profile must not leave behind a setting
+		/// the previous profile made. Local keys in @a file are ignored.
+	inline void importFrom(const QSettings &file, QSettings &live)
+	{
+		const QStringList live_keys = live.allKeys();
+		for (const QString &key : live_keys) {
+			if (!isLocalKey(key)) {
+				live.remove(key);
+			}
+		}
+		const QStringList file_keys = file.allKeys();
+		for (const QString &key : file_keys) {
+			if (!isLocalKey(key)) {
+				live.setValue(key, file.value(key));
+			}
+		}
+		live.sync();
+	}
+}
+
+#endif // CONFIGPROFILE_H