Losts of fixes to cursor handling with insets. Cursor up/down now can

Losts of fixes to cursor handling with insets. Cursor up/down now can
transverse also inset if they are on it's way, Page up/down for InsetText
fixed (cleanly now as I finally understood the algorithm ;) This fixes some
non reported bugs, some in Michaels list, some reported on the mailing
list and #314 on our buglist.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@4008 a592a061-630c-0410-9148-cb99ea01b6c8