Player button mapping UI improvements

[TheRealCatherine]

Right now only spawned players have a button mapping UI (though this may be fixable by having players spawned in edit mode) and only Player1 is supported in the (ugly) button mapping dialog. Additionally there is currently no way other than pressing F1 to open this dialog.

Really player character classes should also return different default buttons based on the player number and should save button mapping information between sessions in the registry, but this can be opened as a separate issue.