GUI - Dropdown improvements

[historic_bruno]

• Implement disabled sprite and text color ("sprite_disabled", "textcolor_disabled")
• Implement hover/mouseover sprite and text color ("sprite_over", "textcolor_over")
• Implement pressed sprite and text color ("sprite_pressed"?, "textcolor_pressed")
• Dynamic size adjustment: currently the max height of the dropdown list is specified with "dropdown_size", it would be nice if it could adjust dynamically to fit in the window. Maybe as an option.
• Jump to the position of the currently selected item, when opening the dropdown. Same problem that CList has.

Dropdowns are a bit weird, the property names are not intuitive or well-documented, here's an explanation I found in styles.xml​:

Dropdown button:
	* size=shape
	* sprite=bkg
	* textcolor=text color
Dropdown arrow button:
	* button_width=shape
	* sprite2=bkg
	* sprite2_pressed=bkg clicked
Dropdown list:
	* buffer_zone=margin
	* dropdown_size=height
	* sprite_list=bkg
	* sprite_selectarea=bkg hovered item
	* textcolor_selected=text color hobered item

See GUI_-_Drop-down.