Improved appearence of regexp insets (at least I hope so), also suggested in #6498...

Improved appearence of regexp insets (at least I hope so), also suggested in #6498 (Regex looks like math).
- added a frame around regexp insets, with a customizable dedicated color code
- regexp insets font turned to texttt
- recovered back the preamble snippet allowing for latex compilation of regexp insets, something that happens if you have instant preview on.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@33329 a592a061-630c-0410-9148-cb99ea01b6c8