git-gui: Allow the user to copy name of the file in the diff viewer.

git-gui: Allow the user to copy name of the file in the diff viewer.

There's a lot of reasons why the user might need to obtain the
complete (or just part of) path of a file which they are currently
viewing in the diff viewer pane.  So now we allow selection on this
widget by using a text widget instead of a label.  We also offer a
context menu which has actions for copying the selection or the entire
value onto the clipboard.

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