UI: Cleanup Qt for Qt6

This changes cleans up some deprecated functions that were removed in
Qt6. Some are placed behind version ifdefs and others are replaced for
their non-deprecated Qt5 version.

UI/window-missing-files.cpp

@@ -68,7 +68,7 @@ QWidget *MissingFilesPathItemDelegate::createEditor(
 	};
 
 	QHBoxLayout *layout = new QHBoxLayout();
-	layout->setMargin(0);
+	layout->setContentsMargins(0, 0, 0, 0);
 	layout->setSpacing(0);
 
 	QLineEdit *text = new QLineEdit();