git-gui: Redesign the display of annotated files.

Using 180 columns worth of screen space to display just 20 columns of
file data and 160 columns worth of annotation information is not
practically useful.  Users need/want to see the file data, and have
the anotation associated with it displayed in a detail pane only when
they have focused on a particular region of the file.

Now our file viewer has a small 10-line high pane below the file
which shows the commit message for the commit this line was blamed
on.  The columns have all been removed, except the current line
number column as that has some real value when trying to locate an
interesting block.

To keep the user entertained we have a progress meter in the status
bar of the viewer which lets them know how many lines have been
annotated, and how much has been completed.  We use a grey background
on the line numbers for lines which we have obtained annotation from,
and we color all lines in the current commit with a yellow background,
so they stand out when scanning through the file.  All other lines
are kept with a white background, making the yellow really pop.

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