Redesigned full-screen diff viewer

Merge Diff class into DiffTextView
ZipRequest: Override server cache headers; calculate own URL
Convert Diff Dialog into an activity (DiffViewer)
DiffViewer: Remove debugging code
Added previous and next button drawables

PS2: Improve whitespace handling
PS5: ZipRequest: Fix decoding problems

Change-Id: Id10b83f479ab9362a73dbea307525b5099f0e40e