More sigkit features

[elbiazo]

What is the feature you'd like to have?
I think there should be better UI for sigkit. I seen a lot of people get confused while using sigkit. I think there should be create and (load/delete/view loaded sig) GUI. Also, it would be cool if we can get current project based signature add whenever Vector35/binaryninja-api#226 lands.

Is your feature request related to a problem?
It is pain to create then put the sig file in binja directory. Maybe there is better way but whenever someone first use binja, it seems like how to use sig kit always get brought up and no one ends up using it.