General cleanup: Text is (or should be) nothing more than InsetText private implement...

General cleanup: Text is (or should be) nothing more than InsetText private implementation. We need access to the owner InsetText property in many cases where we instead take the Paragraph owner inset, which is the same of course. Next step is to avoid this indirection whenever possible.

I also updated InsetMathMBox so that it remains compilable, even if not used.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30940 a592a061-630c-0410-9148-cb99ea01b6c8