Remember the sheet background colour between runs

Picking a sheet (folio) background colour in the diagram editor was lost
on every restart. Diagram::background_color is a static initialised to
white and PaletteGraphicsView's s_custom_bg a static bool, and neither
was ever written anywhere -- Diagram::toXml() carries no colour attribute
either -- so closing and reopening a project always came back on the
default and the choice had to be made again.

Store it in QSettings under diagrameditor/sheet_background_* as a pair of
values rather than one: the colour, and whether it was picked explicitly.
Both halves are needed. "#ffffff, follow the system" and "#ffffff, always
white" are the same colour and two behaviours -- the first is what the
views invert on a dark palette -- so keeping only the colour would
silently turn one into the other on the next start, which is the reported
problem one step removed.

The colour is written as HexRgb on purpose. The SVG export gives
Diagram::background_color an alpha of 0 to render a transparent
background and never puts it back, and that transient value must not be
persisted as a permanently transparent sheet.

Applied from main() after the headless export and scripting branch -- those
return before reaching it and must keep rendering on plain white, the rule
ProjectPrintWindow already enforces for printing -- and before QETApp is
constructed, since that constructor already loads the projects given on
the command line. The GUI export dialog is left alone: it renders through
drawBackground(), so what you see is what you export, as it already was
within a session.

Saved at the moment the colour is applied rather than at shutdown, so
neither the print window's temporary white nor the SVG export's alpha can
reach it. The button's constructor now mirrors the stored state instead of
always claiming "system colour", and the "recently used" list is stored
alongside it.

Covered by tst_sheetbackgroundsetting, which pins the custom flag and the
dropped alpha -- the two rules a single stored colour would lose.

sources/utils/qetsettings.cpp

@@ -152,4 +152,67 @@ namespace QetSettings
 		QSettings settings;
 		settings.setValue("scripting/enabled", enabled);
 	}
+
+	/**
+	* @brief setSheetBackground
+	* Store the sheet background last picked in the diagram editor, so the
+	* next start opens on it for every project, old or new. @sa sheetBackground
+	* @param background
+	*/
+	void setSheetBackground(const SheetBackground &background)
+	{
+		QSettings settings;
+		//HexRgb, not HexArgb, on purpose: Diagram::background_color is
+		//temporarily given an alpha of 0 by the SVG export to make the
+		//background transparent, and that transient value must never end
+		//up here as a stored transparent sheet.
+		settings.setValue(QStringLiteral("diagrameditor/sheet_background_color"),
+				  background.color.name(QColor::HexRgb));
+		settings.setValue(QStringLiteral("diagrameditor/sheet_background_custom"),
+				  background.custom);
+	}
+
+	/**
+	* @brief sheetBackground
+	* @return the sheet background stored by setSheetBackground(), or the
+	* built-in default (white, following the system) when there is none.
+	* @sa setSheetBackground
+	*/
+	SheetBackground sheetBackground()
+	{
+		QSettings settings;
+		SheetBackground background;
+		background.custom = settings.value(
+				QStringLiteral("diagrameditor/sheet_background_custom"),
+				false).toBool();
+		const QColor stored(settings.value(
+				QStringLiteral("diagrameditor/sheet_background_color"),
+				QStringLiteral("#ffffff")).toString());
+		background.color = stored.isValid() ? stored : QColor(Qt::white);
+		return background;
+	}
+
+	/**
+	* @brief sheetBackgroundRecentColors
+	* @return the "recently used" sheet background colours, most recent
+	* first. Empty until the user has picked a non-preset colour.
+	* @sa setSheetBackgroundRecentColors
+	*/
+	QStringList sheetBackgroundRecentColors()
+	{
+		QSettings settings;
+		return settings.value(QStringLiteral("diagrameditor/sheet_background_recent")).toStringList();
+	}
+
+	/**
+	* @brief setSheetBackgroundRecentColors
+	* Store the "recently used" sheet background colours, most recent first.
+	* @sa sheetBackgroundRecentColors
+	* @param colors
+	*/
+	void setSheetBackgroundRecentColors(const QStringList &colors)
+	{
+		QSettings settings;
+		settings.setValue(QStringLiteral("diagrameditor/sheet_background_recent"), colors);
+	}
 }