Extend Thunarx API to be able to set emblems in thunar-plugins

Potential users:

• rabbit-vcs. Looks like for Nautilus that is already possible.
• thunar-vcs-plugin .. see thunar-plugins/thunar-vcs-plugin#13
• thunar-shares-plugin (thunar-plugins/thunar-shares-plugin#2)
• thunar-dropbox
• A possible nextcloud plugin .. see #499