Offline mode can be used even when online.

Allow the user to pick a player name for offline mode.
Big auth refactor. Now using session objects instead of the accounts themselves.
Sessions only last for one instance start and hold all the auth and player data.
Add 'empty text' to all the version selection dialogs.

Customize it for the Forge one so people finally shut up about 1.7.4
Merge branch 'feature_derpstances' of https://github.com/02JanDal/MultiMC5 into feature_derpstances

Conflicts:
	gui/dialogs/OneSixModEditDialog.cpp
	logic/OneSixUpdate.cpp
Fix some bugs that got uncovered while trying to get liteloader 1.7 to work
Merge remote-tracking branch 'upstream/feature_derpstances' into feature_derpstances