git-gui: Install column headers in blame viewer.

git-gui: Install column headers in blame viewer.

I started to get confused about what each column meant in the blame
viewer, and I'm the guy who wrote the code!  So now git-gui hints to
the user about what each column is by drawing headers at the top.
Unfortunately this meant I had to use those dreaded frame objects
which seem to cause so much pain on Windows.

Signed-off-by: Shawn O. Pearce <redacted>