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/docDisplay.js

@@ -88,7 +88,7 @@ DocDisplayItem.prototype = {
 
     // Ensures the preview icon is created.
     _ensurePreviewIconCreated : function() {
-        if (!this._hasPreview || this._previewIcon)
+        if (!this._showPreview || this._previewIcon)
             return; 
 
         this._previewIcon = new Clutter.Texture();