resetting WindowDefault via restart

this is an alternative implementation for #9746, as with me
https://github.com/qgis/QGIS/commit/637359aefc0cd008f749b0fbed140e4cbbf6de15
is not working correctly (it removes processing and other plugins from the
menu's, untill you restart).

this patch adds a QSettings section containng a 'reset' button in Options/System dialog.
upon clicking we write a temporary flag in settings which upon restart makes QGIS clean up the UI settings before setting up it's UI
also added a commandline option: --defaultui doing the same.

I think https://github.com/qgis/QGIS/commit/637359aefc0cd008f749b0fbed140e4cbbf6de15 could be reverted, if others also conclude that it has unwanted side effects.