snap: use the same swiftshader branch as the emugl project does

This gives us various fixes needed to get a working graphical experience
with the swiftshader OpenGL ES driver.

Limit snap architectures to amd64

We only support the snap for amd64 at the moment.

Add basic touchscreen support

Various changes and fixes torwards 18.04 support

Add installation instructions

The used profile is derived from what LXD uses to confine its
containers. See https://github.com/lxc/lxd/blob/master/lxd/apparmor.go

Upstream release
https://github.com/bombela/backward-cpp/archive/v1.3.tar.gz

This implements client side window decorations of our windows. The title
bar of our windows is now drawn by Android and we simply map out the
title bar area hit test area to SDL to detect when the other wants to
move the window or resize it.

dbus-cpp was dropped from 18.04 as it is not maintained anymore so we
should migrate to something more stable with a long term perspective.
libsystemd fits in nicely here as it has a nice and clean dbus
implementation which is battle tested and can be easily integrated into
our existing C++ code base.

Systems with GLVND load the actual EGL implementation in a different way
when the GL driver is dlopen'ed and therefor we need to tell GLVND which
vendor implementations are available on the system. As snapd is not yet
exposing this to the snap we have to ship a small set of configs on our
own until that is fixed.