Update screen as needed when preferences are changed

Update screen as needed when preferences are changed

This commit removes the adhoc code in GuiPrefs and moves it to LyXRC,
so that it triggers in every preference change.

The code has also been updated to trigger on more variables, e.g.
font_*_foundry.

Note that the actual function that are called have not been changed
(addPreviews and LFUN_SCREEN_FONT_UPDATE), although there are doubts
that they behave as needed (see FIXMEs).

Fixes bug #11498.