For LMMS files to be clickable in a Linux desktop environment, there
need to be a program accepting such files as an argument. This patch
change the lmms desktop file to affect files to open on the command line.
I did not know if lmms accepted URLs to remote files, so I went with the
safer %f for local files.
This patch originated in the Debian packaging, see for example
<URL: https://sources.debian.net/src/lmms/1.0.3-5/debian/patches/ >.
Cleans up the root directory of the GitHub source tree and starts to separate platform-specific installing and packaging logic from the master CMakeLists.txt. Closes#2201