Ported decade popup logic to a controller
Various changes:
* Changed the widgets hierarchy; now the PopupButton and ImageToggleButton are based on the OptionsButton, that represents a button which various options connected to a controller that provides those options and which is the responsible of doing an action on the model when the UI gets certain signals.
* Modified the ImageToggleButton to accept more than two states. Not really necesary but it could come handy sometime.
* Added the create_pixbuf_from_file_at_size on the utils module. This function returns a pixbuf with the given width and height, created from a file. The reason to add this function is that the Pixbuf's constructor 'new_from_file_at_size' tries to respect the image scale, so if the given width and height aren't in the same scale (eg 1:2), one of the two gets ignored.
* Ported the logic of the SortOrderToggleButton to a controller, modifying all the necesary code for it to work.
Changed the name of the item_selected method for option_selected, since it's a somewhat more general name