Expose favourite tags via API

[AlexCSDev]

Would be nice to see an API function which will return all favourite tags. When you have a lot of them it starts to become pretty hard to understand what is what when you don't have any kind of a graphical preview. I wanted to make an app for that, but unfortunately currently Hydrus doesn't privide any API for retrieving favourite tags.

[floogulinc]

This would also work to prime the tag autofill in Hydrus Web with your favorite tags. Good idea.