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/cmake-extras/qmlplugindump_path.patch

@@ -0,0 +1,13 @@
+diff --git a/src/QmlPlugins/QmlPluginsConfig.cmake b/src/QmlPlugins/QmlPluginsConfig.cmake
+index f563330..7e326bf 100644
+--- a/src/QmlPlugins/QmlPluginsConfig.cmake
++++ b/src/QmlPlugins/QmlPluginsConfig.cmake
+@@ -2,7 +2,7 @@
+ # target before loading this plugin.
+ 
+ if(NOT TARGET qmlplugindump)
+-    find_program(qmlplugindump_exe qmlplugindump)
++    find_program(qmlplugindump_exe qmlplugindump HINTS /usr/lib/qt5/bin/)
+ 
+     if(NOT qmlplugindump_exe)
+       msg(FATAL_ERROR "Could not locate qmlplugindump.")