* Remove register keyword
Register is meaningless in c++, and removed in c++17
* Fix compiler warningsg
Add missing override, remove unused capture, and remove unused local variablees
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.
