Refactoring fields of Window for clarity

[StanleyGoldman]

• There is an enum called SubTab with members History, Changes, etc.
• There are several sub classes of SubView: HistoryView, BranchesView, etc.

Window has a member HistoryTab et al. which is an instance of HistoryView et al.
It's rather confusing, I'm renaming them to be the same as the type, which isn't great but better than the -Tab suffix I felt.
The alternative would be to name them History et al.

[StanleyGoldman]

#252 Depends on this

[StanleyGoldman]

#261 Depends on this

[shana]

Everything looks good apart from the Window.cs.orig file that was mistakenly pushed along with the rest of the stuff.

[StanleyGoldman]

Rebased

Failure condtions have changed