[Gtk4Prep] Cleanup DnD

[jeremypw]

• Move DnD utils into one file
• Avoid unnecessary use of Gdk.DragContext (which does not exist in Gtk4)

Incorporates #2417 to facilitate testing

[danirabbit]

I have a few errors here:

../src/View/Widgets/BreadcrumbsEntry.vala:81.33-81.45: error: Argument 1: Cannot convert from `void Files.View.Chrome.BreadcrumbsEntry.on_drag_leave (uint)' to `delegate void Gtk.Widget.drag_leave (Gtk.Widget, Gdk.DragContext, uint)'
   81 |             drag_leave.connect (on_drag_leave);
      |                                 ^~~~~~~~~~~~~  
../src/View/Widgets/BreadcrumbsEntry.vala:314.32-314.38: error: Argument 1: Cannot convert from `unowned Gdk.DragContext' to `uint'
  314 |                 on_drag_leave (context, timestamp);
      |                                ^~~~~~~             
../src/View/Widgets/BreadcrumbsEntry.vala:314.17-314.50: error: 1 extra arguments for `void Files.View.Chrome.BreadcrumbsEntry.on_drag_leave (uint)'
  314 |                 on_drag_leave (context, timestamp);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
../src/View/AbstractDirectoryView.vala:725.40-725.52: error: Argument 1: Cannot convert from `void Files.AbstractDirectoryView.on_drag_leave (uint)' to `delegate void Gtk.Widget.drag_leave (Gtk.Widget, Gdk.DragContext, uint)'
  725 |             widget.drag_leave.connect (on_drag_leave);

[jeremypw]

@danirabbit Ooops! Don't know how I missed that.

[jeremypw]

@danirabbit Thanks for the review - I'll work on the bugs. Presumably caused by switching to EventControllers.