gitk: Make a row of controls for controlling highlighting

gitk: Make a row of controls for controlling highlighting

Now there is a bar across the middle (just below the bar containing
the sha1 ID, find string etc.) which controls highlighting.  There are
three ways to highlight: the user can highlight commits affecting
a list of paths, commits in a view, or commits where the author or
committer matches any of a list of strings (case-insensitive).  The
elements of the list of paths and list of names are delimited by
whitespace with shell quoting rules.

Signed-off-by: Paul Mackerras <paulus@samba.org>