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/unity-api/qmltestrunner.patch

@@ -0,0 +1,13 @@
+diff --git a/cmake/modules/QmlTest.cmake b/cmake/modules/QmlTest.cmake
+index 8241703..d209b55 100644
+--- a/cmake/modules/QmlTest.cmake
++++ b/cmake/modules/QmlTest.cmake
+@@ -15,7 +15,7 @@
+ # qmltest_DEFAULT_IMPORT_PATHS
+ # qmltest_DEFAULT_PROPERTIES
+ 
+-find_program(qmltestrunner_exe qmltestrunner)
++find_program(qmltestrunner_exe qmltestrunner-qt5)
+ 
+ if(NOT qmltestrunner_exe)
+   message(FATAL_ERROR "Could not locate qmltestrunner.")