add backwards compatibility for previous pre-releases.

whiners.
better fix for tu check on psi attack

because we don't pass a reference or a pointer or something to the state,
i don't know.
there must be a better fix than this, but this works so whatever.
make more use of the forward/back mouse buttons

- add a selectPreviousPlayerUnit() function in BattleScapeState, for use with "back"
- allow for checking reselect flag on existing selectPreviousPlayerUnit function in SavedBattleGame
- use forward and back buttons in inventory state
- use forward and back buttons in unit info state
Fixed errors in research tree.
More research bugs
- Fixed digital music not loading if GM.cat is present
- Fixed missing support for extra PSX tracks
Whoops
inlining SavedBattleGame::getTile()

it's the most-called method in the whole project
Merge branch 'master' of https://github.com/SupSuper/OpenXcom

Conflicts:
	src/Savegame/SavedGame.cpp