Refactor diff colors; Move GerritSearchView

Make the TextView in DiffDialog its own view and have it handle
 its colouring/formating
Extract diff colours to attributes so they can change based on theme,
 improving diff readablity
Move GerritSearchView to a new Views package
Diff and DiffDialog refactoring

Change-Id: I7223cc94b74382357a194b53440efbce7a56c188