Integration Improvements

Styling:

• Added item label font and color customization through FatSidebarItemStyle
• Added item removal delegate callback
• Added editFatSidebarItem(_:) IBAction for Interface Builder wirings, e.g. in contextual menus

Selection handling:

• Added public visibility to selectedItems
• Added deselectAllItems() as a convenience command
• Added selectItem(at:) in case you don't keep the item references around
• Added item toggle (in addition to, not as a replacement of selection!) delegate callback

Fixes:

• Fixed double-clicking an item triggering a (de)selection after a slight delay
• Fixed quickly moving mouse away from item not hiding the hover overlay if its animation did not yet finish