build: port to gtksourceview-4

[rbuj]

based on https://gitlab.gnome.org/GNOME/gtksourceview/-/commit/54e48e9c further info https://developer.gnome.org/gtksourceview/stable/porting-guide-3-to-4.html closes #618

[mbkma]

gtksourceview4 is not present in Debian 10, so I can not test this at the moment, because I get:

configure: error: Package requirements (
    libxml-2.0 >= 2.5.0
    glib-2.0 >= 2.50.0
    gmodule-2.0
    gthread-2.0 >= 2.13.0
    gio-2.0 >= 2.50.0
    gtk+-3.0 >= 3.22.0
    gtksourceview-4 >= 4.0.2
    libpeas-1.0 >= 1.2.0
    libpeas-gtk-1.0 >= 1.2.0
) were not met:

No package 'gtksourceview-4' found

gtksourceview4 is in Debian testing: https://tracker.debian.org/pkg/gtksourceview4

[rbuj]

@mbkma libgtksourceview-4-dev is available from buster

[kloczek]

Just tested that PR and looks like at least ljust released plum builds with that :)

[raveit65]

@rbuj @mbkma Anything else to merge which we need for pluma-plugins before i make a new release and update travis (ci) config?

[rbuj]

I think #617 should be merged before releasing a new version of pluma.