Add an option to keep the folio row and column headers visible (#1034)

Affichage > "Garder les en-têtes visibles" adds a bar along the top and
the left of the diagram view that repeats the folio's column numbers and
row letters, aligned with the cells at any zoom, so they stay in sight
when the folio's own headers are scrolled away. Off by default; the
choice is stored as diagrameditor/cell_rulers.

The bars are CellRuler widgets in the view's margins
(setViewportMargins), not scene items, so printing and PDF/PNG/DXF
export never see them, and they paint with the application palette
outside of the dark-palette inversion. They keep a constant thickness;
when cells get narrower than their labels, only every 2nd, 5th, 10th...
label is written. A bar is hidden when the folio hides that header.
Showing or hiding them keeps the centre of the view where it was.

The labels come from BorderCellLabels, now also used by
BorderTitleBlock::draw(), so the bars and the border cannot disagree.
PNG export of all 133 folios of the examples is pixel-identical to
master, with border-columns_0 true and false.

Known limits: changing border-columns_0 repaints the bars at the next
scroll or zoom; the menu toggle updates the views of its own editor
window only, like the grid toggle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G2d2Zi8BfrYRPX88zhaoFG

sources/bordertitleblock.cpp

@@ -17,6 +17,7 @@
 */
 #include "bordertitleblock.h"
 
+#include "bordercelllabels.h"
 #include "createdxf.h"
 #include "diagram.h"
 #include "diagramposition.h"
@@ -533,6 +534,8 @@ void BorderTitleBlock::draw(QPainter *painter)
 
 		//Draw the nums of columns
 	if (display_border_ && display_columns_) {
+		const bool columns_start_at_zero =
+				settings.value("border-columns_0", true).toBool();
 		for (int i = 1 ; i <= columns_count_ ; ++ i) {
 			QRectF numbered_rectangle = QRectF(
 				diagram_rect_.topLeft().x()
@@ -543,23 +546,15 @@ void BorderTitleBlock::draw(QPainter *painter)
 				columns_header_height_
 			);
 			painter -> drawRect(numbered_rectangle);
-			if (settings.value("border-columns_0", true).toBool()){
 			painter -> drawText(numbered_rectangle,
 					    Qt::AlignVCenter
 					    | Qt::AlignCenter,
-					    QString("%1").arg(i - 1));
-			}else{
-			painter -> drawText(numbered_rectangle,
-					    Qt::AlignVCenter
-					    | Qt::AlignCenter,
-					    QString("%1").arg(i));
-			}
+					    BorderCellLabels::columnLabel(i, columns_start_at_zero));
 		}
 	}
 
 		//Draw the nums of rows
 	if (display_border_ && display_rows_) {
-		QString row_string("A");
 		for (int i = 1 ; i <= rows_count_ ; ++ i) {
 			QRectF lettered_rectangle = QRectF(
 				diagram_rect_.topLeft().x(),
@@ -575,8 +570,7 @@ void BorderTitleBlock::draw(QPainter *painter)
 			painter -> drawText(lettered_rectangle,
 					    Qt::AlignVCenter
 					    | Qt::AlignCenter,
-					    row_string);
-			row_string = incrementLetters(row_string);
+					    BorderCellLabels::rowLabel(i));
 		}
 	}