Port to GTK5

[paolostivanin]

https://gnome.pages.gitlab.gnome.org/gtk/gtk/gtk-migrating-3-to-4.htm

https://developer.gnome.org/gtk4/3.98/migrating.html

[paolostivanin]

src/ui/otpclient.ui:121:          <object class="GtkButtonBox">
src/ui/otpclient.ui:256:          <object class="GtkButtonBox">
src/ui/otpclient.ui:349:          <object class="GtkButtonBox">
src/ui/otpclient.ui:411:          <object class="GtkButtonBox">
src/ui/otpclient.ui:575:          <object class="GtkButtonBox">
src/ui/otpclient.ui:882:          <object class="GtkButtonBox">
src/ui/otpclient.ui:1001:          <object class="GtkButtonBox">

[paolostivanin]

(OTPClient:8511): GLib-GObject-WARNING **: 22:49:47.368: The property GtkWidget:margin-left is deprecated and shouldn't be used anymore. It will be removed in a future version.

(OTPClient:8511): GLib-GObject-WARNING **: 22:49:47.368: The property GtkWidget:margin-right is deprecated and shouldn't be used anymore. It will be removed in a future version.

(OTPClient:8511): GLib-GObject-WARNING **: 22:49:47.370: The property GtkAlignment:left-padding is deprecated and shouldn't be used anymore. It will be removed in a future version.

(OTPClient:8511): GLib-GObject-WARNING **: 22:49:47.372: The property GtkContainer:child is deprecated and shouldn't be used anymore. It will be removed in a future version.

[paolostivanin]

gtk_widget_show_all() has been removed, widgets are visible by default now

[paolostivanin]

adapt to all api changes (gtkbutton, gtkheaderbar, etc)

[paolostivanin]

This might be postponed for v3.0 in case GTK4 won't be declared stable before the end of 2019

[paolostivanin]

Given that v2 might come out before the end of summer 2019 and GTK4 is not even close on being declared stable, I have decided to postpone the port to v3.0.0