Use return for confirmation in SinglePlayerMenu.

I also added a way for any window to handle Return keys by overriding the on_return method in Window.
In Multiplayer Menu, make Return join the selected game, if any.
Use return for confirmation in the Editor Menu.
Fix typo.
Support on_return for ingame windows.
Merge pull request #2094 from unknown-horizons/single_player_dialog

Add a way to handle Return events in windows