rfc: find dialog replaced with findbar

[Stephen Kennedy <stevek gnome org>]

http://svn.gnome.org/viewvc/meld?view=revision&revision=1264

I've replace the find dialog with a findbar and added replace support
at the same time.
The filediff.glade diff looks huge but it's really just changing
filediff widget into a vbox
and adding the bar at the end.

I've tried to keep the bar as small as possible while maintaining the
current features.
Suggestions welcome.

Stephen.