Add draggable window title bars

Implements drag-to-reorder for window title bars, following up on the
merged window title bar feature (#9450) and the design discussion in #9619.

- Drag a title bar and drop on another title bar to swap positions
- Drop on a window body quadrant (left/right/top/bottom) to insert as
  a directional split; Splits layout uses insert_window_next_to(), other
  layouts fall back to move_window_to_group()
- Drop on a tab bar tab to move the window into that tab
- Drop on another OS window to move into its active tab
- Drop outside kitty to detach into a new OS window
- Tab bar highlights the hovered tab during a window drag, mirroring
  how the destination window title bar is highlighted
- toggle_window_title_bars action temporarily force-shows title bars
  for drag-to-reorder when they are normally hidden, auto-hiding after
  the drag completes
- window_title_bar_drag_threshold option (default 5px) controls how far
  the mouse must move before a drag is initiated; 0 disables dragging

MIME type follows the same convention as tab dragging:
application/net.kovidgoyal.kitty-window-{PID}

Ref: #9619

kitty/options/definition.py

@@ -1550,6 +1550,16 @@ opt('window_title_bar_align', 'center',
     choices=('left', 'center', 'right'),
     long_text='Horizontal alignment of the text in window title bars.'
     )
+
+opt('window_title_bar_drag_threshold', '5',
+    option_type='positive_int',
+    long_text='''
+Pixel distance the mouse must move before a window title bar drag begins.
+Zero disables dragging. Drop on a title bar swaps positions; drop on a
+window body inserts in the quadrant direction (left/right/top/bottom).
+Drop on the tab bar moves the window to that tab; drop outside kitty
+detaches it to a new OS window.
+''')
 egr()  # }}}
 
 

kitty/options/parse.py

@@ -1529,6 +1529,9 @@ class Parser:
 
     choices_for_window_title_bar_align = choices_for_tab_bar_align
 
+    def window_title_bar_drag_threshold(self, val: str, ans: dict[str, typing.Any]) -> None:
+        ans['window_title_bar_drag_threshold'] = positive_int(val)
+
     def window_title_bar_inactive_background(self, val: str, ans: dict[str, typing.Any]) -> None:
         ans['window_title_bar_inactive_background'] = to_color_or_none(val)
 

kitty/options/types.py

@@ -504,6 +504,7 @@ option_names = (
     'window_title_bar_active_background',
     'window_title_bar_active_foreground',
     'window_title_bar_align',
+    'window_title_bar_drag_threshold',
     'window_title_bar_inactive_background',
     'window_title_bar_inactive_foreground',
     'window_title_bar_min_windows',
@@ -705,6 +706,7 @@ class Options:
     window_title_bar_active_background: kitty.fast_data_types.Color | None = None
     window_title_bar_active_foreground: kitty.fast_data_types.Color | None = None
     window_title_bar_align: choices_for_window_title_bar_align = 'center'
+    window_title_bar_drag_threshold: int = 5
     window_title_bar_inactive_background: kitty.fast_data_types.Color | None = None
     window_title_bar_inactive_foreground: kitty.fast_data_types.Color | None = None
     window_title_bar_min_windows: int = 0