* src/configdata.{cc,h}: Remove config option "menutool_mode" because it's
crack now that we have undo et al.  Add config options "textview_font",
"match_color" and "current_match_color".

* src/filebuffer.{cc,h} (pango_context_changed): Assign default font to the
error_message tag.
([gs]et_match_color): New accessors to tag colors.
([gs]et_current_color): ditto.

* src/mainwindow.{cc,h} ([gs]et_menutool_mode): Get rid of these.
(on_filetree_switch_buffer): No longer modify the TextView font globally
since we set the tag now.
(on_preferences): Handle the added config settings.
(load_configuration): ditto
(save_configuration): ditto.

* src/prefdialog.{cc,h} (FontSelectionButton): New utility widget.
(ColorSelectionButton): Another new utility widget.
(PrefDialog): Revamp the preferences dialog.  That is, add GUI for the new
config settings and remove the old one.  Also replace the radio buttons for
selecting the toolbar style with an option menu.