Plugin improvements

The location of the database plugin has been moved to the new lib/plugin
folder.  Additionally, the Wavefront object and material parser has been
removed from the base opengl library and changed to be a separate
plugin.  It has also been updated to better match the ECS ExportAttr
specifications.

This commit also changes the .tagged?() method in the EntityManager to
accept multiple tags.  The method will return false unless all tags
specified are present.