Fix license headers and comment
Add Key header, containing keysym defs
Implement DragController interface
Allow non-header-guarded output from keysymgen
Clean up key/mouse event argument semantics
Implement bitset based on enums
Make the EnumBitset interface more similar to std::bitset
Implement initializer_list inferface for EnumBitset
Migrate CSSState to new EnumBitset class template