- 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/metadata.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
+<pkgmetadata>
+	<maintainer type="project">
+		<email>proaudio@gentoo.org</email>
+		<name>Gentoo ProAudio Project</name>
+	</maintainer>
+	<use>
+		<flag name="hid">Enable HID controller support</flag>
+		<flag name="shout">Enable broadcasting support via <pkg>media-libs/libshout</pkg></flag>
+		<flag name="vamp">Enable vamp analyzing plugins support</flag>
+	</use>
+	<upstream>
+		<remote-id type="sourceforge">mixxx</remote-id>
+	</upstream>
+</pkgmetadata>