Add an option to draw the folio cell limits across the drawing

Affichage › "Afficher les limites des cases" draws a faint dashed line
at every column and row limit of the folio border, so that zoomed into
the middle of a folio you can see where a cell ends, not only which one
the headers name. Off by default, remembered once set
(diagrameditor/cell_lines).

Drawn by DiagramView::drawBackground(), under every item and only in
the view: printing and export render the Diagram, never the view, so
they cannot pick the lines up. PaletteGraphicsView gains scenePainter()
so a subclass can draw there on the inverted (dark palette) path too.

The lines follow the border's own cell positions, and each direction is
hidden when the folio hides that header.

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

sources/diagramview.cpp

@@ -30,6 +30,7 @@
 #include "qetgraphicsitem/conductortextitem.h"
 #include "qetgraphicsitem/independenttextitem.h"
 #include "qeticons.h"
+#include "qetpalette.h"
 #include "titleblock/integrationmovetemplateshandler.h"
 #include "ui/diagrampropertiesdialog.h"
 #include "ui/multipastedialog.h"
@@ -112,6 +113,7 @@ DiagramView::DiagramView(Diagram *diagram, QWidget *parent) :
 	m_top_ruler = new CellRuler(Qt::Horizontal, this);
 	m_side_ruler = new CellRuler(Qt::Vertical, this);
 	m_cell_rulers_shown = QSettings().value("diagrameditor/cell_rulers", false).toBool();
+	m_cell_lines_shown = QSettings().value("diagrameditor/cell_lines", false).toBool();
 	connect(&m_diagram->border_and_titleblock, &BorderTitleBlock::borderChanged, this, &DiagramView::updateCellRulers);
 	connect(&m_diagram->border_and_titleblock, &BorderTitleBlock::displayChanged, this, &DiagramView::updateCellRulers);
 	updateCellRulers();
@@ -1196,6 +1198,72 @@ void DiagramView::paintingInverted(bool inverted)
 	m_diagram->setInvertedLightness(inverted);
 }
 
+/**
+	@brief DiagramView::setCellLinesShown
+	Show or hide the lines that mark the columns and the rows of the folio
+	border across the drawing, in this view only: printing and exporting
+	never draw them.
+	@param shown
+*/
+void DiagramView::setCellLinesShown(bool shown)
+{
+	m_cell_lines_shown = shown;
+	viewport()->update();
+}
+
+/**
+	@brief DiagramView::drawBackground
+	Reimplemented from PaletteGraphicsView: over the folio background, the
+	cell lines when they are shown. Dashed and faint, so they do not read
+	as conductors, and under every item.
+	@param painter
+	@param rect
+*/
+void DiagramView::drawBackground(QPainter *painter, const QRectF &rect)
+{
+	PaletteGraphicsView::drawBackground(painter, rect);
+
+	const BorderTitleBlock &border = m_diagram->border_and_titleblock;
+	if (!m_cell_lines_shown || !border.borderIsDisplayed()) {
+		return;
+	}
+
+		//Where the border draws its cells, whether or not the other
+		//header is displayed
+	const QPointF origin(Diagram::margin + border.rowsHeaderWidth(),
+			     Diagram::margin + border.columnsHeaderHeight());
+	const qreal right = origin.x() + border.columnsCount() * border.columnsWidth();
+	const qreal bottom = origin.y() + border.rowsCount() * border.rowsHeight();
+
+	QPainter *p = scenePainter(painter);
+	p->save();
+	p->setRenderHint(QPainter::Antialiasing, false);
+	QColor color = QET::Palette::gridDotColor(Diagram::background_color,
+						  invertsLightness());
+	color.setAlpha(70);
+	QPen pen(color, 1, Qt::DashLine);
+	pen.setCosmetic(true);
+	p->setPen(pen);
+
+	if (border.columnsAreDisplayed()) {
+		for (int i = 1 ; i < border.columnsCount() ; ++i) {
+			const qreal x = origin.x() + i * border.columnsWidth();
+			if (x >= rect.left() && x <= rect.right()) {
+				p->drawLine(QPointF(x, origin.y()), QPointF(x, bottom));
+			}
+		}
+	}
+	if (border.rowsAreDisplayed()) {
+		for (int i = 1 ; i < border.rowsCount() ; ++i) {
+			const qreal y = origin.y() + i * border.rowsHeight();
+			if (y >= rect.top() && y <= rect.bottom()) {
+				p->drawLine(QPointF(origin.x(), y), QPointF(right, y));
+			}
+		}
+	}
+	p->restore();
+}
+
 /**
 	@brief DiagramView::setCellRulersShown
 	Show or hide the rulers that keep the column numbers and the row