The character dialog was disabled in r30810 for solving an assertion

The character dialog was disabled in r30810 for solving an assertion
due to a missing buffer and because the "dialog is only useful in texted".
The problem with the buffer member has been solved since then, and while
it is true that the character dialog is not much useful in mathed, it
is the only way for coloring only parts of equations. Given that this
would also be a regression with respect to 1.6, I am re-enabling it.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@34327 a592a061-630c-0410-9148-cb99ea01b6c8