MDL-69087 added an option to let themes personalize the H5P styles.
This patch implements the h5p_alter_styles() method in boost, to
let admins personalize the H5P styles using the 'Raw initial SCSS'
and 'Raw SCSS' theme settings.
That way, users won't need to create their own themes to define
some styles for the H5P player.
A new plugintype has been created for having more than one installed
third-party H5P libraries. Existing libraries have been moved from
lib/h5p to the new h5plib_v124 plugin.
