EnhancedIconview: made some fixes on the do_button_press_event method.

* The item-clicked event now send the event two
* The call to the parent class do_button_press_event is made after our custom behavior
* Fixed typo (variable brought from the old code).
CoverartSource: refactored the click callback using the new capabilities of the EnhancedIconview.

Selecting the path on right click and showing the popup is taken care on the new iconview, the only left to do in the source is the second click detection to show/hide the bottom pane.