gitk: support auto-copy comit ID to primary clipboard

gitk: support auto-copy comit ID to primary clipboard

Auto-select ("Copy commit ID to X11 selection") is useful when a
selection cliboard exists, but otherwise generally meaningless, for
instance on Windows.

Add a similar pref and behavior which copies the commit ID to the
primary clipboard - for platforms without a selection clipboard, but
which can also be useful additionally on platforms with selection.

Note that while autoselect is enabled by default, autocopy isn't.

That's because the selection clipboard is typically dispensable, while
the primary clipboard can be considered a more precious resource,
which we don't want to (clear and) overwrite by default.

Signed-off-by: Avi Halachmi (:avih) <avihpit@yahoo.com>