Absolute plugin path gets compiled into binary

[lukas-w]

Currently the absolute path to the plugin load directory is compiled into the binary (can be set by running cmake -DCMAKE_INSTALL_PREFIX=…)

This has two implications:

1. Moving the lmms installation to a different location on the file system is not possible without recompiling. (Without recompiling errors such in Building LMMS errors #32 may occur)
2. After building, installing lmms to a different prefix than previously specified through cmake also requires a recompile.

LMMS should therefore instead store a path relative to the main executable.

[lukas-w]

Maybe it should generally be possible to specify a plugin search path in the settings.