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

glfw/cocoa_window.m

@@ -909,14 +909,15 @@ static void update_titlebar_button_visibility_after_fullscreen_transition(_GLFWw
         self.identifier = @"kitty-content-view";
 
         [self updateTrackingAreas];
-        char tab_mime[64];
+        char tab_mime[64], window_mime[64];
         snprintf(tab_mime, sizeof(tab_mime), "application/net.kovidgoyal.kitty-tab-%d", getpid());
+        snprintf(window_mime, sizeof(window_mime), "application/net.kovidgoyal.kitty-window-%d", getpid());
         NSMutableArray *types = [NSMutableArray arrayWithObjects:
             NSPasteboardTypeFileURL, NSPasteboardTypeString, NSPasteboardTypeURL, NSPasteboardTypeColor,
             NSPasteboardTypeFont, NSPasteboardTypeHTML, NSPasteboardTypePDF, NSPasteboardTypePNG,
             NSPasteboardTypeRTF, NSPasteboardTypeSound, NSPasteboardTypeTIFF,
             UTTypeData.identifier, UTTypeItem.identifier, UTTypeContent.identifier,
-            mime_to_uti(tab_mime),
+            mime_to_uti(tab_mime), mime_to_uti(window_mime),
         nil];
         // Add file promise types
         [types addObjectsFromArray:[NSFilePromiseReceiver readableDraggedTypes]];
@@ -1591,6 +1592,10 @@ update_drop_state(_GLFWwindow *window, size_t mime_count) {
         for (size_t i = 0; i < d->mimes_count; i++)
             _glfwPlatformRequestDropData(window, d->mimes[i]);
     }
+    // Restore first-responder status after native DnD; the drag operation can
+    // displace the content view from first responder, silently breaking keyboard
+    // input even though osw->is_focused remains true.
+    [window->ns.object makeFirstResponder:window->ns.view];
     return YES;
 }