git-gui: Support keyboard traversal in browser.

git-gui: Support keyboard traversal in browser.

Users want to navigate the file list shown in our branch browser
windows using the keyboard.  So we now support basic traversal
with the arrow keys:

  Up/Down:  Move the "selection bar" to focus on a different name.

  Return:   Move into the subtree, or open the annotated file.
  M1-Right: Ditto.

  M1-Up:    Move to the parent tree.
  M1-Left:  Ditto.

Probably the only feature missing from this is to key a leading part
of the file name and jump directly to that file (or subtree).

This change did require a bit of refactoring, to pull the navigation
logic out of the mouse click procedure and into more generic routines
which can also be used in bindings.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>