This singleton class handles management of plugin search paths and plugin
discovery. Search paths are (if they exist):
* <lmms-exe-dir>/../lib/lmms: This is the common location on Unixoids
(Not included in Windows builds)
* <lmms-exe-dir>/plugins: For portable and Windows installations
* The path given by the compile define LMMS_PLUGIN_DIR
* Environment variable LMMS_PLUGIN_DIR if given
This commit also tweaks the build script to output built plugins to
"${CMAKE_BINARY_DIR}/plugins". This way lmms can find plugins during
development without the need to use `make install`.
Plugin::getDescriptorsOfAvailPlugins and ConfigManager::pluginDir were
removed.
