[202_2723]: Add toolbar entries for overbrace, underbrace and underline

[dorodb-web22]

Fixes #2723

What

Added 3 missing entries to the wide accent toolbar popup in math mode:

• \overbrace (make-wide)
• \underbrace (make-wide-under)
• \underline / wide-bar (make-wide-under)

Also added the corresponding 16x16 XPM icons using the same 8-color palette
as the existing accent icons (e.g. tm_bar.xpm).

Changed Files

• TeXmacs/progs/math/math-menu.scm — 3 new entries in the wide accent popup
• TeXmacs/misc/pixmaps/modern/16x16/focus/tm_wide_overbrace.xpm — new icon
• TeXmacs/misc/pixmaps/modern/16x16/focus/tm_wide_underbrace.xpm — new icon
• TeXmacs/misc/pixmaps/modern/16x16/focus/tm_wide_bar.xpm — new icon
• devel/202_2723.md — dev doc

Known Issue

I used the EXACT SAME COLOR PALETTE and a pixel art as the existing icons for the 3 new XPM icons ,

but they render slightly differently at small sizes
(the a appears as a blue dot instead of a clear letter).

I wasn't able to figure out why ... the pixel data is byte-for-byte identical to the
working icons. Happy to take suggestions on improving the icons.

Testing

• Built with xmake build stem, ran with xmake run stem
• Entered math mode, opened the wide accent popup
• Verified all 3 new entries appear and insert the correct constructs

[JackYansongLi]

The font for overbrace underbrace looks wierd to me. It is more like a hat symbol.