settings: fix videoscreen setting changing once and for all

This should be the final fix for the dreaded videoscreen setting changes which are/were a complete mess because they all influence each other. But, as always, in the end the solution was much simpler than expected. Instead of trying to equally handle changes to videoscreen.screen/resolution/screenmode we now only really change the display configuration when videoscreen.screenmode is changed. This is the one that is used throughout the rest of the xbmc code and the other two are just there to show them as GUI settings and changing them will always result in changing videoscreen.screenmode. That also greatly simplifies the problem of not really knowing when to show the "Would you like to keep this resolution?" prompt.

@FernetMenta: With this changing refreshrates should finally be fixed as well.