Luca Bacci authored Feb 20, 2019 and Mike Fleetwood committed Apr 27, 2019

The Gdk::RGBA data type was introduced to replace Gdk::Color in
Gtkmm 3.0 [1], with Gdk::Color being deprecated in Gtkmm 3.10 [2].

With this commit we make the change to Gdk::RGBA data type which is the
modern replacement to Gdk::Color.  Gdk::RGBA can be used almost as a
drop in replacement because it keeps most of the Gdk::Color interface.

Also, this commit removes the C Gtk call introduced during the
port-to-gtk3 patchset by commit:
    53793527
    repare-for-gtk3: Prepare for removal of Gtk::Widget::modify_fg() (#7)

[1] Gtkmm 3.0.1 NEWS file
    https://gitlab.gnome.org/GNOME/gtkmm/blob/3.0.1/NEWS#L48
        * RGBA replaces Color, though Color still exists because it is
          used by TextView.  We hope to deprecated Color completely in
          gtkmm 3.2.

[2] Gtkmm 3.10.0 NEWS file
    https://gitlab.gnome.org/GNOME/gtkmm/blob/3.10.0/NEWS#L127
        Gdk:
        * Deprecate Color.

Closes !25 - Modern Gtk3 - part 1