Initial implementation of the quick entry on a overlay for the iconview
Added functionality to the quick search (issue #99). TODO: timeout to hide the quick searcn entry
Minor bugfixes:
- Prevent the quick search to gain focus if a modifier key is being pressed, since they have an special meaning on the cover view.
- Move the cover view cursor to the album selected by the quick search mechanism.
Bugfix: instead of removing and readding an item into the sorted collection, defined a reorder method which internally checks if the items needs to be reordered and return the new index in case it does (issue #119). Removed the pre-modified signal since it's no longer needed.
Bugfix: the toolbar positions radiobuttons were firing an extra signal when the pref window was opened, forcing the toolbar to dissapear if it was positioned in one of the sidebars. Changed the radiobuttons for a more suitable (in my opinion) combobox, fixing this bug at the same time (issue #117).
Clean the widgets module:
* Some minor style adjustments following python PEPs
* Removed the superflous ```initial_label``` PopupButton's property and its accesors, since it was confusing and didn't add any real value. The value saved on it was something only used on the subclasses (the superclass only used it on initialization) and is really easy to access it using other mediums instead (other object properties). Plus, using accesors on python isn't really necesary. For plain variables there aren't needed at all, and if you need to do some proccessing before setting/getting a variable, you can replace accesors with a property.
* Fixed bug on the SortPopupButton that caused that the radio button would, on initialization, select the 'Sort by album name' item, even if that wasn't the current sorting key.
Added a visible border around the text quick search entry to make it blend better with it's surroundings (issue #99)
Added a timeout of 3 seconds after the last input on the quick search entry before automatically hiding it (issue #99).
Hide the quick search entry when it loses focus
Fixed bug on ReversedSortedCollection class' index method
Added the posibility to navigate through the quick search matches using the up and down arrows. (issue #99)
Change timeout from 3 to 4 seconds and fix an issue with the match selection (issue #99)
bug fix - remove use of deprecated VBOX
Merge branch 'quick-search'

This introduces the quick search feature to the master branch. This feature allows the user to do a quick search by album name through the visible albums, quickly selecting the first match and allowing the user to navigate through the matches using the up and down arrow keys (close #99)
add new button plus code for decade filter - issue #120
add new icon - issue #120
remove orig file - issue #120
add decade spritesheet and code - issue #120
revised decade spritesheet - issue #120
minor calculation changes on spritesheet