git-gui: Refactor options menu into an options dialog.

git-gui: Refactor options menu into an options dialog.

I decided that the options menu was going to turn into a mess after
a while as I start to add additional features to git-gui.  The better
approach would be to create a dialog that lets the user edit the options,
including their --global options.

We also wisely let the user press Cancel (or destroy the window) to abort
any sort of option editing session, without the options being changed.

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