Merge player_t into interface_t. Rename player_sett_t to player_t.

Merge player_t and interface_t into one struct called interface_t. This struct shall contain only the stuff that is related to the interface to the human player.
Removed updates of button event bit field in interface_t. Use gui events instead.
viewport: Handle double click using gui events. Replaces the old method of checking bit flags which is not valid anymore.
Rename player_sett_t -> player_t.