unity8: initial packaging (!27)

* Mir starts up and is able to display system settings
* x86_64 only for now, because at least ubuntu-app-test did not build
  on aarch64

Based on PureTryOut's work. Getting it to this stage was a huge effort
(as it shows in the package count: 111(!)). See the merge request for
details.

[skip ci]: this won't finish in CI; ollieparanoid made sure that
           everything builds for x86_64.

unity8/ubuntu-ui-toolkit/replace-bash-for-sh.patch

@@ -0,0 +1,10 @@
+diff --git a/documentation/docs.sh b/documentation/docs.sh
+index 10fcfc49d..aa93dea62 100755
+--- a/documentation/docs.sh
++++ b/documentation/docs.sh
+@@ -1,4 +1,4 @@
+-#!/bin/bash
++#!/bin/sh
+ #
+ # Copyright 2012 Canonical Ltd.
+ #