The old context menu implementation adds the thesaurus
sub-menu by manipulating the menu at runtime, which isn't a
good idea in general. Since it's a sub-menu anyway, better
to have it as a separate controller, so it can be added
to the xml, and users could decide if they want it, and
where.

Most of the code adapted from sfx2 (menu/mnumgr.cxx,
menu/thessubmenu.cxx), hence the Apache-based license
header.

Change-Id: I4f533fcdd5d6480fae8ebcf53ec7c69675025adb