git-gui: Move console procs into their own namespace

git-gui: Move console procs into their own namespace

To help modularize git-gui better I'm isolating the code and
variables required to handle our little console windows into
their own namespace.  This way we can say console::new rather
than new_console, and the hidden internal procs to create the
window and read data from our filehandle are off in their own
private little land, where most users don't see them.

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