Add adjustable background frame to the element editor

Adds a toggleable, visual-only background frame to help proportion an
element's drawing against a representative folio surface, instead of
guessing against a bare grid. Follows the exact precedent of the
existing hotspot indicator (ElementScene::drawForeground()): drawn by
ElementView::drawBackground(), centered on the same origin, and never
written to the saved .elmt file.

- "Afficher le cadre de fond" toggles the frame on/off.
- "Taille du cadre de fond..." opens a small width/height dialog to
  resize it, defaulting to a representative folio's drawable area
  derived from BorderProperties::defaultProperties() (columns_count *
  columns_width by rows_count * rows_height).
- Both the visibility and the last-used size persist via QSettings,
  the same way the existing grid-dot rendering prefs
  (elementeditor/grid_pointsize_min/max) already do.

Implements the scope proposed in discussion #604.

sources/editor/elementview.cpp

@@ -598,6 +598,19 @@ void ElementView::drawBackground(QPainter *p, const QRectF &r) {
 			}
 		}
 	}
+
+	if (m_scene->backgroundFrameVisible()) {
+		const QSizeF frame_size = m_scene->backgroundFrameSize();
+		const QRectF frame_rect(-frame_size.width() / 2.0, -frame_size.height() / 2.0,
+					 frame_size.width(), frame_size.height());
+		QPen frame_pen(Qt::blue);
+		frame_pen.setCosmetic(true);
+		frame_pen.setStyle(Qt::DashLine);
+		p -> setPen(frame_pen);
+		p -> setBrush(Qt::NoBrush);
+		p -> drawRect(frame_rect);
+	}
+
 	p -> restore();
 }