Allow custom overlays in buttonbar

[Stiltzkin]

current behaviour:
right now you get an overlay icon in the bottom right corner of the icon if you set the "[x] show as menu".
if you want slight variations, for lets say a folder icon, you have to create each icon manually.

suggestion:

• an option to set a custom overlay icon at a specific position (left corner, right corner, middle, ....) for buttonbar icons (sometimes called 'badge' oder 'bullet' icon).

and/or

• the ability to customize the existing overlay icon for "[x] show as menu"