workspaceThumbnail: Expose maxThumbnailScale as property

Some extensions want to modify the value of the MAX_THUMBNAIL_SCALE
constant. That is no longer possible, as exports are always read-only
from the outside.

Make this possible again by exposing the scale as a property on the
object itself, so extensions can override it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2939>

js/ui/overviewControls.js

@@ -195,7 +195,7 @@ class ControlsManagerLayout extends Clutter.BoxLayout {
                 this._workspacesThumbnails.get_preferred_height(width);
             thumbnailsHeight = Math.min(
                 thumbnailsHeight * expandFraction,
-                height * WorkspaceThumbnail.MAX_THUMBNAIL_SCALE);
+                height * this._workspacesThumbnails.maxThumbnailScale);
             childBox.set_origin(0, startY + searchHeight + spacing);
             childBox.set_size(width, thumbnailsHeight);
             this._workspacesThumbnails.allocate(childBox);