Minor refactor of diff code to prepare for unified view

Pulled some classes out of SideBySide2. There will be a lot of
common code between the upcoming Unified2 and SideBySide2.

Fixed a minor bug in skip line rendering. It was still relying
on Side.PARENT. Changed to use DisplaySide.A.

Deleted deprecated PatchSelectBox2.

Change-Id: I87d608d7e6a4330096d65b0fc1b99c1c5160d119
13 files changed