Add tooltip delay to the preview pop-ups.

Showing previews after a delay allows the user to move the mouse around
the screen without triggering constant pop-ups.

Make sure we remove the pop-up when the user hits Escape and redisplay
the pop-up if we are updating the section results due to a change in space
allocated for it.

Rename protected variable _hasPreview to _showPreview in order to not have
the naming conflict with a new private variable _havePointer, which we
name in first person.

js/ui/overlay.js

@@ -146,6 +146,11 @@ Sideshow.prototype = {
         this._searchEntry.connect('key-press-event', function (se, e) {
             let symbol = Shell.get_event_key_symbol(e);
             if (symbol == Clutter.Escape) {
+                // We always want to hide the previews when the user hits Escape.
+                // If something that should have a preview gets displayed under 
+                // the mouse pointer afterwards the preview will get redisplayed.
+                me._appDisplay.hidePreview(); 
+                me._docDisplay.hidePreview(); 
                 // Escape will keep clearing things back to the desktop.  First, if
                 // we have active text, we remove it.
                 if (me._searchEntry.text != '')