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
Updated Polish translation
mumble, mumble_exe: fix overlay self-detection for the mumble_app.dll build.

When Mumble was built as the mumble.exe/mumble_app.dll pair, mumble_ol.dll's
overlay self-detection logic didn't work, because it failed to find the
special "mumbleSelfDetection" symbol.

This change adds that symbol to mumble_exe. It is perhaps not the cleanest
solution to the problem, but doing it this way allows older mumble_ol.dlls
to continue to work with mumble_app.dll builds, and we avoid changing the
overlay self-detection check in mumble_ol.dll.

Fixes issue #1053.