git-gui: Refactor some UI init to occur earlier

git-gui: Refactor some UI init to occur earlier

I'm starting to setup a main window that the user can use to
locate an existing repository, clone an existing repository,
or create a new repository from scratch.  To help do that I
want most of our common UI support already defined before we
start to look for the Git repository, this way if it was not
found we can open a window to help the user locate it.

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