Gtk3 minimum required version

[Thiago Milczarek Sayão]

Hi,

Currently the minimum required version is 3.8.

Beginning on 3.10 Gtk added support for client-side decorations and
the GtkHeaderBar is the default widget to do that.

https://docs.gtk.org/gtk3/ctor.HeaderBar.new.html

Gtk 3.10 was released Sep/2013.

Gtk4 and Gnome apps default to this behaviour.

By using it we may be able to get rid of the window sizing problem on
glass-gtk which is "decoration sizes" thru _NET_FRAME_EXTENTS which is
very error prone (it depends on each window manager).
Currently it does a lot of work-arounds to size the window properly
because the server does the decoration so there is a problem getting
it's size to size the window accordingly. It works, but it's far from
pretty.

More details here:
https://discourse.gnome.org/t/understanding-window-sizing-and-decorations-on-gtk4/7290

By moving to this idea we might be more compatible with other apps and
do a better job on the implementation of window sizing.

If you put a pure gtk window and a javafx window on Ubuntu 20.04 there
are some differences (shadow size and titlebar size).

Also, if we want to support wayland we will have to require a greater
gtk3 version. Or just support it on Gtk4 where I would follow the
HeaderBar (client-side decorations) idea.

-- Thiago.