offer fallback from HiDPI on low resolution displays

[2387skju]

In case of unsuitable scale factor set: Offer the user an easy way to switch back.

Issue situation:

• If the User set HiDPI on a low resolution display, its hard to revert this change.
• Thats because of, the respective UI are now not visible anymore.
• The most Users don't know how handle this problem (=to make the UI parts visible):
  • move a window by hotkey: ALT + Mouse-click
  • not possible in another way (=> not possible by right click on Window-Decoration-Header/Title -> move window )

I've added a new dialog ( Screenshot from 1280 x 800 pixel monitor ):

Dialog as Text

Title: confirm window scaling factor Text: Do you want to keep this new setting? ( Tip: Move windows by press and hold: ALT + Mouse-Click (left) )

The dialog will be only shown, when

• change to Hi-DPI
• if the Mate-Tweak window will not fit in new screen size (rought)
• it check the current used monitor. In later version of GDK (>3), it will only check the first monitor.