Write the list of refused dropped files with an English colon

The message listing the files a drop could not add wrote each line as
"name : reason", with the space French puts before a colon, and as a
fixed string that no translation could change. It is now a tr() text,
"name: reason", so each language can write it its own way.

Suggested in the review of #1416.

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

sources/diagramview.cpp

@@ -455,7 +455,7 @@ void DiagramView::handleImageFilesDrop(QDropEvent *e)
 		QString error;
 		const QImage image = ImageDrop::load(file, &error);
 		if (image.isNull())
-			refused << QStringLiteral("%1 : %2").arg(QFileInfo(file).fileName(), error);
+			refused << tr("%1: %2").arg(QFileInfo(file).fileName(), error);
 		else
 			items << new DiagramImageItem(QPixmap::fromImage(image));
 	}