git-gui: Consolidate the Fetch and Push menus into a Remote menu

git-gui: Consolidate the Fetch and Push menus into a Remote menu

Sometimes the Fetch menu looks really odd, such as if you are in a
repository that has no remotes configured when you start git-gui.
Here we didn't have any items to add to the Fetch menu so it was a
tad confusing for the end-user to see an empty menu on the menu bar.

We now place all of the commands related to fetching and pushing of
changes into a single "Remote" menu.  This way we have a better class
of bucket that we can drop additional remote related items into such
as doing a remote merge or editing the remote configuration specs.

The shortcuts to execute fetch/remote prune/push on existing remote
specifications are now actually submenus listing the remotes by name.

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