Draft: DirectoryMergeWindow as a dockable widget
This MR implements https://bugs.kde.org/show_bug.cgi?id=439640.
Also, it fixes https://bugs.kde.org/show_bug.cgi?id=439676 but in a naive way.
DirectoryMergeWindow and DirectoryMergeWidget are placed inside a QDockWidget to make them movable and fix the frustrating space waste. The DockWidgets are not closable to match the current behavior. Maybe the Action "Folder & Text Split Sreen View" should be re-thought if using dock widget. Another solution could be to add an action to revert the QSplitters... I forked because for me, it's much easier to work with kdiff3 with this GUI layout.
I think the discussion should be opened about a customizable layout. This MR is to provide a preview of such a feature =)