[StWidget] add (optional) hover tracking

If track-hover is set, update the hover property automatically, and
the "hover" pseudo class to match, as StClickable used to do. (Remove
the corresponding code in StClickable). Tweak the tooltip handling to
use track-hover, which also makes it slightly more reliable in the
presence of reactive children, etc.

js/ui/appDisplay.js

@@ -536,6 +536,8 @@ AppWellIcon.prototype = {
     },
 
     _onMenuPoppedDown: function() {
+        this.actor.sync_hover();
+
         if (this._didActivateWindow)
             return;
         if (!this._setWindowSelection)

js/ui/workspacesView.js

@@ -484,10 +484,7 @@ NewWorkspaceArea.prototype = {
     },
 
     setStyle: function(isHover) {
-        if (isHover)
-            this._child1.add_style_pseudo_class('hover');
-        else
-            this._child1.remove_style_pseudo_class('hover');
+        this._child1.set_hover(isHover);
     }
 };