* ui/prefdialog.glade (button_textview_font): Replace custom font
selection button with the new GtkFontButton.
(button_match_color): Replace custom color selection button with
the new GtkColorButton.
(button_current_color): ditto.
(combo_toolbar_style): Rename from option_toolbar_style.  Use the
new GtkComboBox widget instead of GtkOptionMenu.

* src/prefdialog.{cc,h} (FontSelectionButton): Remove.
(ColorSelectionButton): Remove.
(PrefDialog): Accomodate to changes in the glade file.
(on_textview_font_set): Rename from on_textview_font_selected().
(on_match_color_set): Rename from on_match_color_selected().
(on_current_color_set): Rename from on_current_color_selected().