Allow installation of plugins not only in ~/.config

Used by app-misc/openrgb-plugin-* packages

--- a/PluginManager.cpp
+++ b/PluginManager.cpp
@@ -35,7 +35,8 @@ void PluginManager::ScanAndLoadPlugins()
     | The plugins directory is a directory named "plugins" in   |
     | the configuration directory                               |
     \*---------------------------------------------------------*/
-    const QDir plugins_dir = QString().fromStdString(ResourceManager::get()->GetConfigurationDirectory()) + "plugins/";
+    for (const QDir plugins_dir : {QString().fromStdString(ResourceManager::get()->GetConfigurationDirectory()) + "plugins/",
+                                   QString().fromStdString(GENTOO_PLUGINS_DIR)}) {
 
     /*---------------------------------------------------------*\
     | Get a list of all files in the plugins directory          |
@@ -56,6 +57,7 @@ void PluginManager::ScanAndLoadPlugins()
 
         AddPlugin(plugin_path);
     }
+    }
 }
 
 void PluginManager::AddPlugin(std::string path)