Did some fixes on the EnhancedIconView

* The do_size_allocate wasn't calling the superclass behavior, and the allocation wasn't being correctly made.
* Added a signal to inform when a single click is made on a icon.
Replaced the IconView for the EnhancedIconView as the cover_view on the source ui
Added the object_column property to the EnhancedIconView on the source ui
Had to call to the superclass' do_button_press_event too, otherwise the selection wouldn't work
Use the available variables instead of saving redundant information
Removed the update_iconview_callback method from the source since it's not needed anymore
Added a timeout to the columns reallocation to avoid innecesary reallocations while the user is adjusting the pane size
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.
Gonna start moving up those methods that got refactored to make it easier to identify those that still need to be looked at
Removed a signal that isn't used anymore
CoverartSource: fixed a issue with the second click bottom pane expand. It wasn't working correctly after a shift or ctrl multi selection
EnhancedIconview: fixed the get_selected_objects methods, it wasn't managing the model correctly
CoverartSource: replaced the get_selected_albums method for the new get_selected_objects method of the EnhancedIconview class
EnhancedIconview: refactored the code to select and scroll to a path on the iconview.

* Moved the code to select and scroll to the a path from the source to the iconview.
* Refactored the AlbumQuickSearchController to use this new capability directly insted of through the source.
Merge branch 'enhanced-iconview'

Introduced the EnhancedIconview widget, which enhances the iconview capability in some senses:
* Adds a signal for a single click on a icon.
* Fixes column reallocation on width change.
* Adds a popup to the widget, that gets fired when right clicking on an icon.
* Adds a new property to indicate the column of the model that contains the actual object to which the icon and other info belongs to. Through this property, a new method get_selected_objects allows to retrieve those objects corresponding to the current selection on the iconview.
* Adds a method to select and scroll to a given path on the iconview.