- media-sound/mixxx: re-add mixxx to fix Qt5 issues

Mixxx is compiled with Qt5 in ~gx86 and some core features are broken
with Qt5, so we need to backport those following patches.

- https://github.com/mixxxdj/mixxx/pull/845/commits
- https://github.com/mixxxdj/mixxx/pull/848/commits

media-sound/mixxx/files/mixxx-2.0.0-docs.patch

@@ -0,0 +1,10 @@
+--- mixxx-9999/src/SConscript
++++ mixxx-9999/src/SConscript
+@@ -290,7 +290,6 @@
+                 env.Alias('install', controllermappings)
+                 env.Alias('install', translations)
+                 env.Alias('install', keyboardmappings)
+-                env.Alias('install', docs)
+                 env.Alias('install', dotdesktop)
+                 env.Alias('install', icon)
+                 env.Alias('install', promotracks)