Use 'f' in a patch to browse the list of files in the patch set

When the user presses 'f' inside of a patch viewer we now open up
the list of patches from the same patch set, and let the user move
through them with the keyboard.  Selecting a file opens it in the
viewer, but leaves the dialog open until they trigger it to close.

Bug: GERRIT-136
Signed-off-by: Shawn O. Pearce <sop@google.com>
20 files changed