git-gui: Convert browser, console to "class" format

git-gui: Convert browser, console to "class" format

Now that we have a slightly easier method of working with per-widget
data we should make use of that technique in our browser and console
meta-widgets, as both have a decent amount of information that they
store on a per-widget basis and our current approach of handling
it is difficult to follow.

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