Add an option to export symbols as DXF blocks (#1339)

With "DXF : symboles en blocs" ticked in the export dialog, or
--dxf-blocks on the command line, each symbol definition a folio uses is
written once as a DXF block and every placed symbol as an INSERT of it.
A CAD program then selects, counts and replaces a symbol as one object.

The block is drawn by the same code as a symbol drawn in full, as if
the symbol sat unturned at the folio's origin, which is DXF (0,
sheetHeight): that is the block's base point, so the INSERT carries the
symbol's position and quarter-turn and nothing else. Blocks have to come
before any entity, so the export collects them first and dxfBegin()
writes them in its BLOCKS section. Entities in a block keep their own
layers. Off by default; with it off the file is unchanged.

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

sources/exportproperties.cpp

@@ -37,6 +37,7 @@ ExportProperties::ExportProperties() :
 	draw_terminal_names(false),
 	draw_bg_transparent(false),
 	draw_colored_conductors(true),
+	dxf_blocks(false),
 	exported_area(QET::BorderArea)
 {
 }
@@ -77,6 +78,8 @@ void ExportProperties::toSettings(QSettings &settings,
 			  draw_bg_transparent);
 	settings.setValue(prefix % "drawcoloredconductors",
 			  draw_colored_conductors);
+	settings.setValue(prefix % "dxfblocks",
+			  dxf_blocks);
 	settings.setValue(prefix % "area",
 			  QET::diagramAreaToString(exported_area));
 }
@@ -115,6 +118,8 @@ void ExportProperties::fromSettings(QSettings &settings,
 	draw_colored_conductors = settings.value(
 				prefix % "drawcoloredconductors",
 				true ).toBool();
+	dxf_blocks = settings.value(prefix % "dxfblocks",
+				    false).toBool();
 
 	exported_area  = QET::diagramAreaFromString(
 				settings.value(