Eliminate static images

I previously had static UIImages declared, which is obviously not ideal
(especially for less-used ones). Instead, they are now static strings
with locations, with which I can easily use UIImage.FromFile, while
still keeping the code fairly clean.
Refactoring

When I stepped back at my virtually feature-complete project, I noticed
one main thing: there is tons of duplicated code. This commit
introduces PBViewController, which is a good start towards removing
duplicate code. This makes the code much more maintainable, because a
simple change that affects many files can now be fixed in once place.
Hopefully I can refactor away some of the back button logic, as well as
rotation-related code.