Slot symbol editing

[Chr1Z93]

This introduces a new button on the playermat which will allow you to click on a slot and select a new symbol for it (e.g. to get an additional ally slot because you have Charisma etc.)

Also includes a clean up of custom UI assets (removal of unused ones) and some improvements to Nkosi

closes #650

Maybe interesting features to add at a later point:

• allow setting multiple slots (probably by right-clicking to append, maybe selection if it's an "or" or an "and" connection)
• allow user to enter comment