patch 9.2.0338: Cannot handle mouseclicks in the tabline

patch 9.2.0338: Cannot handle mouseclicks in the tabline

Problem:  Cannot handle mouseclicks in the tabline
Solution: Support %[FuncName] click regions in 'tabline', add "area" key
          to the click info dict (Yasuhiro Matsumoto).

The previous implementation resolved and stored click regions only for
per-window statuslines; the tabline path in win_redr_custom() (wp==NULL)
parsed %[FuncName] but discarded the regions, and tabline clicks were
dispatched via TabPageIdxs[] which didn't know about them.

Add a global tabline_stl_click array populated from the tabline path,
refactor stl_click_handler() to take the regions directly, and dispatch
matching clicks from do_mouse() before falling through to tab selection.
The winid entry in the callback dict is 0 for tabline clicks.

related: #19841
closes:  #19950

Supported by AI.

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>