git-gui: Allow the user to copy the version data to the clipboard.

git-gui: Allow the user to copy the version data to the clipboard.

If a user wants to report an issue they will likely want to include
the version number with their issue report.  This may be difficult
to enter if the version number includes an abbreviated commit SHA1
on the end of it.  So we now give the user a context menu option
on the version box which allows them to copy all of the relevant
version data to the clipboard, ready for pasting into a report.

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