mainwindow: add an option to avoid restoring window geometry

Avoid restoring window geometry when `restore_window_geometry=false`
in `~/.config/nvim-qt/window-geometry.conf`. Defaults to `true`.

Suggested-by: @damanis on github
Related-to: #997
Related-to: #1094

src/gui/mainwindow.cpp

@@ -312,6 +312,8 @@ Shell* MainWindow::shell()
 void MainWindow::saveWindowGeometry()
 {
 	QSettings settings("nvim-qt", "window-geometry");
+	bool restore_window_geometry = settings.value("restore_window_geometry", true).toBool();
+	settings.setValue("restore_window_geometry", restore_window_geometry);
 	settings.setValue("window_geometry", saveGeometry());
 	settings.setValue("window_state", saveState());
 }
@@ -325,7 +327,9 @@ void MainWindow::restoreWindowGeometry()
 #endif
 
 	QSettings settings("nvim-qt", "window-geometry");
-	restoreGeometry(settings.value("window_geometry").toByteArray());
+	if (settings.value("restore_window_geometry", true).toBool()) {
+		restoreGeometry(settings.value("window_geometry").toByteArray());
+	}
 	restoreState(settings.value("window_state").toByteArray());
 }