collection-filter-switcher: Convert buttons to GtkRadioButton
GtkRadioButton is an specialization of GtkToggleButton that allows for only one of the group to be selected. This way we avoid the bug when a second click in the filter switcher untoggles a button and leaves them all untoggled (which is inconsistent with the view state indicated by the switcher itself).
Setting "draw-indicator" for each of the buttons allows them to have the visual aspect of a normal GtkToggleButton.
Fixes #354 (closed)