Add extra search path for third party plugins

The OBSBasic UI will now allow the use of a subdirectory of the user
application data directory for third-party plugins.  Though I'm not
entirely sure if this ideal or not.  Regardless, this is one of the
first (of many) steps towards a plugin manager.

On windows, this is %appdata%/obs-studio/plugins
On linux, this is ~/.obs-studio/plugins
On mac, this is ~/Library/Application Support/plugins