Keep the previous versions of a project when saving it

Saving replaced the project file with nothing kept of what it was: one
bad save -- a fault in QElectroTech, a full disk, a wrong edit saved by
mistake -- and the last good version was gone. AutoCAD keeps a .bak on
every save, KiCad several backups.

Before a project is saved over, the file as it was is now copied to a
folder beside it, "<name>-backups", named by the time that version was
saved ("industrial 2026-10-16 08-30-12.qetz"); only the newest are kept,
5 by default (Settings > General > Projects, 0 for none). A copy is
never a move, so a save that fails leaves the file in place; a copy that
fails only warns, the save goes on. The same bytes as the newest version
kept are not kept twice.

The prompt on opening ("Create a backup copy?"), which writes one dated
copy beside the project and never removes any, is left as it is.

New tst_previousversions (newest kept, same bytes once, same second,
nothing for a new file or a count of 0, a failed copy said); it fails
with the oldest never removed. Checked with the program: two scripted
saves of a .qetz leave the original and the first save in
industrial-backups, the project holds the second. ctest 107/107.

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

sources/utils/previousversions.h

@@ -0,0 +1,49 @@
+/*
+	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 PREVIOUSVERSIONS_H
+#define PREVIOUSVERSIONS_H
+
+#include <QString>
+#include <QStringList>
+
+/**
+	@brief The PreviousVersions namespace
+	The versions of a project file kept before it is saved over, as KiCad
+	and AutoCAD keep theirs: a copy of the file as it was goes in a folder
+	beside it, "<name>-backups", named by the time that version was saved
+	("industrial 2026-10-16 08-30-12.qetz"), and only the newest are kept.
+	The file is copied, never moved: a save that fails leaves it in place.
+*/
+namespace PreviousVersions
+{
+		/// How many versions to keep (Settings > General > Projects); 0 keeps none
+	int count();
+	void setCount(int count);
+		/// The folder the versions of @p file_path go in
+	QString folder(const QString &file_path);
+		/// The versions kept of @p file_path, newest first
+	QStringList kept(const QString &file_path);
+		/// Copy @p file_path, about to be saved over, among its versions and
+		/// keep only the @p count newest. Nothing is copied when the file does
+		/// not exist, when @p count is 0, or when the newest version kept
+		/// holds the same bytes. @return false, with @p error, when the copy
+		/// could not be made: the save goes on all the same.
+	bool keep(const QString &file_path, int count, QString *error = nullptr);
+}
+
+#endif // PREVIOUSVERSIONS_H