FlxUIDropDownMenu improvements
- text now shows the first entry of the DataList by default instead of "item 1"
- added destroy()
- more intuitive order of constructor params, added a description
- center the text that displays the current item vertically
- fix highlight graphic exceeding the borders with default assets
- the button now acts as a toggle button instead of only opening the menu
- you now have to click somewhere outside the of the menu to close it, as opposed to not hovering over it anymore closes it