Multi instance preferences handling

Follow-up to Bug #1638678 "screen display 0.92pre2 vs other viewers and 0.91".

From comment #8:

"...when you change the preferences in one window, the change won't take effect, when you switch back to the other document, and that overwrites the preferences again.

(I guess it has them in memory, and saves when changed, doesn't read directly before saving - and even then, there could be a gap...).
No restart can change that ;-) I noticed this when switching languages to review translations."

Possible improvements:
 - warn users when an other instance modified the preferences;
 - synchronize the preferences file for all open instances when closing an instance.