patch 9.1.1834: MS-Windows: not possible to highlight the title bar

Problem:  MS-Windows: not possible to highlight the title bar
Solution: Make the title/caption bar configurable by introducing the
          'go-C' option value which allows to highlight it using the
          TitleBar and TitleBarNC highlighting groups (Mao-Yining).

Introduce titlebar color customization for Windows 11 GUI through
highlight groups and new 'guioptions' flag:

- Add 'C' flag to enable titlebar color customization (opt-in)
- New highlight groups: TitleBar (active) and TitleBarNC (inactive)
- Uses DWMWA_CAPTION_COLOR and DWMWA_TEXT_COLOR DWM attributes
- Dynamically loads dwmapi.dll for Windows 11 compatibility
- Defaults to system colors when set to NONE or feature disabled

closes: #18449

Signed-off-by: Mao-Yining <mao.yining@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/gui_w32.txt

@@ -1,4 +1,4 @@
-*gui_w32.txt*   For Vim version 9.1.  Last change: 2025 Aug 06
+*gui_w32.txt*   For Vim version 9.1.  Last change: 2025 Oct 07
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -488,4 +488,20 @@ VIM_KEYCODE_TRANS_STRATEGY can be set to the desired value ("experimental" or
         set VIM_KEYCODE_TRANS_STRATEGY=experimental
         gvim.exe
 <
+
+Title Bar						*gui-w32-title-bar*
+
+Starting with Windows 11, you can customize the gVim title bar (also known as
+the caption bar) by enabling the |'go-C'| option.
+
+Once enabled, the appearance is controlled by two highlighting groups:
+1. |hl-TitleBar|   -- Sets the color of the title bar for the active window.
+2. |hl-TitleBarNC| -- Sets the color of the title bar for inactive windows.
+
+To use the system's default title bar colors, set highlighting groups to
+`NONE`: >
+
+	hi TitleBar guibg=NONE guifg=NONE
+	hi TitleBarNC guibg=NONE guifg=NONE
+
  vim:tw=78:sw=4:ts=8:noet:ft=help:norl: