Allow keyboard control to work in the staging widgets.

Allow keyboard control to work in the staging widgets.

Keyboard focus was restricted to the commit message widget and users were
forced to use the mouse to select files in the workdir widget and only then
could use a key combination to stage the file.
It is now possible to use key navigation (Ctrl-Tab, arrow keys and Ctrl-T
or Ctrl-U) to stage and unstage files.
Suggested by @koppor in git-for-window/git issue #859

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>