git-gui: Add a search command to the blame viewer.

git-gui: Add a search command to the blame viewer.

One of the largest deficiencies in the blame viewer at
the moment is the impossibility to search for a text
string. This commit fixes it by adding a Firefox-like
search panel to the viewer.

The panel can be shown by pressing F7 or clicking a
menu entry, and is hidden by pressing Esc. Find Next
is available through the F3 key.

Implementation is based on the gitk code, but heavily
refactored. It now also supports case-insensitive
searches, and uses the text box background color to
signal success or failure of the search.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>