Improve `--quit` option: second try

!35 (merged) was the first try, essentially reverted by !49 (merged). This time, the procedure is simpler and should be less controversial: a simple loop on the MousepadWindows list, trying to close them by triggering the dedicated action (thus trying to save, etc.), and aborting at the first failure (typically an undo by the user when asked to save a modified document).

This still requires some additional modifications to ensure that all possible dialogs are "gracefully destroyed with parent", but the option does nothing more than what is said above (it does not modify, in particular, the application windows list).

A classical menu entry is also added to the "File" menu, assigned Ctrl+Q since this is so conventional, and reassigning Ctrl+Shift+W to "Close Window" (also quite conventional).