Merge branch 'terencehill/listbox_item_highlight' into 'master'

Listbox: highlight item under the cursor

* Highlight listbox focused items with a customizable (by skin attributes) fade effect; the same effect is applied to the crosshair picker and to the charmap too

* Crosshair picker now has customizable (by skin attributes) color and alpha, similarly to the charmap ones

* The dot crosshair now appears always white in the crosshair picker

* The crosshair preview is no longer highlighted, nor clickable as it's no longer a button (made no sense)

About the implementation:
Changed the crosshair picker and the crosshair preview from a set of special radio buttons to 2 new different items: a picker with code based on the charmap code and a preview. More precisely the crosshair picker extends the new Picker superclass (same for the charmap).

See merge request !122