git-gui: Convert blame to the "class" way of doing things

git-gui: Convert blame to the "class" way of doing things

Our blame viewer code has historically been a mess simply
because the data for multiple viewers was all crammed into
a single pair of Tcl arrays.  This made the code hard to
read and even harder to maintain.

Now that we have a slightly better way of tracking the data
for our "meta-widgets" we can make use of it here in the
blame viewer to cleanup the code and make it easier to work
with long term.

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