Multipolygon annotation tool

[rsimon]

This has been brought up as a feature request multiple times. A tool for creating an annotation target consisting of multiple polygon shapes. There's nothing that prevents this in principle. Just needs thinking through the interaction design (and writing the code :-)

Probably, this is best implemented as a separate drawing tool plugin.

[rsimon]

Also moving this from the core Annotorious repo to the selector pack.

[erikradisch]

13

[rsimon]

Done! 🎉