workspace: Remove WindowOverlay in favour of new overlays of WindowClone

Start using the new overlays we introduced in the last commit and remove
the WindowOverlay class and the objects for keeping track of them in the
Workspace.

The new layout which doesn't use the -shell-close-overlap CSS property
anymore sligthly changes the position of the close button to be a bit
further away from the actual window.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1298

data/theme/gnome-shell-sass/widgets/_window-picker.scss

@@ -8,6 +8,7 @@ $window_thumbnail_border_color:transparentize($selected_fg_color, 0.65);
 $window_close_button_size: 24px;
 $window_close_button_padding: 3px;
 
+$window_clone_border_size: 6px;
 
 // Window picker
 .window-picker {
@@ -22,7 +23,7 @@ $window_close_button_padding: 3px;
 
 // Borders on window thumbnails
 .window-clone-border {
-  border-width: 6px;
+  border-width: $window_clone_border_size;
   border-style: solid;
   border-color: $window_thumbnail_border_color;
   border-radius: $base_border_radius + 2;
@@ -34,6 +35,7 @@ $window_close_button_padding: 3px;
 
 // Window titles
 .window-caption {
+  margin-top: $window_clone_border_size / 2;
   color: $osd_fg_color;
   background-color: $osd_bg_color;
   border:1px solid $osd_outer_borders_color;
@@ -54,8 +56,6 @@ $window_close_button_padding: 3px;
   width: $window_close_button_size;
   box-shadow: -1px 1px 5px 0px rgba(0,0,0,0.5);
 
-  -shell-close-overlap: $window_close_button_size * 0.5;
-
   &:hover {
     background-color: lighten($selected_bg_color, 5%);
   }