Add x-scheme-handler support to the Mumble .desktop file.

This is the modern way to handle application-specific URL schemes
on the various desktop environments on Unix-like systems.

Fixes #1061 (reported by Chris Knadle; thanks!)

Originally fixed in Ubuntu via
https://bugs.launchpad.net/ubuntu/+source/mumble/+bug/934239

scripts/mumble.desktop

@@ -8,10 +8,11 @@ GenericName[tr]=Sesle Sohbet
 Comment=A low-latency, high quality voice chat program for gaming
 Comment[fr]=Un logiciel de chat vocal de haute qualité et de faible latence pour les jeux
 Comment[tr]=Oyunlar için yüksek kaliteli ve düşük gecikmeli sesle sohbet programı
-Exec=mumble
+Exec=mumble %u
 Icon=mumble
 Terminal=false
 Type=Application
 StartupNotify=false
+MimeType=x-scheme-handler/mumble
 Categories=Network;Chat;Qt;
 Version=1.0