Fixes #2723
Add corresponding hints/prompts for overbrace, underbrace, and underline in the math mode toolbar.

Summary

Added direct options for overbrace, underbrace and underline under math mode toolbar. Also created and added .xpm and .svg icon sets for them.

Developer Document: devel/222_52.md

Issue found

The accent toolbar had no direct options for overbrace, underbrace, or underline even though these constructs are supported by math insertion and keyboard bindings. This made them harder to discover and inconsistent with other accent operations.

Changes

In TeXmacs/progs/math/math-menu.scm:

+ ((balloon (icon "tm_overbrace.xpm") "keyboard equivalent:") (make-wide "<wide-overbrace>"))
+ ((balloon (icon "tm_underbrace.xpm") "keyboard equivalent:") (make-wide-under "<wide-underbrace>"))
+ ((balloon (icon "tm_underbar.xpm") "keyboard equivalent:") (make-wide-under "<wide-bar>"))))

In TeXmacs/misc/pixmaps/modern/16x16/focus added these files:

• tm_overbrace.xpm
• tm_underbrace.xpm
• tm_underbar.xpm

In TeXmacs/misc/pixmaps/liii/16x16/focus added these files:

• tm_overbrace.svg
• tm_underbrace.svg
• tm_underbar.svg

In TeXmacs/misc/pixmaps/liii-night/16x16/focus added these files:

• tm_overbrace.svg
• tm_underbrace.svg
• tm_underbar.svg

How to Test

1. Open Mogan Editor.
2. Enter math mode ($).
3. Open the accent dropdown from the tm_wide icon.
4. Verify new entries are present with dedicated icons:
   • overbrace
   • underbrace
   • underline (underbar)
5. Click each entry and verify insertion result:
   • overbrace inserts a wide overbrace construct
   • underbrace inserts a wide underbrace construct
   • underline inserts wide underbar (\underline behavior)
6. Hover the new entries and verify tooltip formatting matches other accent items.

Before:

After: