KeyMap is now switched (if necessary) only just before it is actually used

KeyMap is now switched (if necessary) only just before it is actually used
for translating keystrokes. Up until now, the switching was done in advance
in all kinds of situations, but not just before translating was actually done
--- which resulted in the KeyMap not always being switched on time...

This solves bug #3811 (http://bugzilla.lyx.org/show_bug.cgi?id=3811)

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18744 a592a061-630c-0410-9148-cb99ea01b6c8