Implement multi-select

[pbrada]

I have worked with a couple of diagrams. The multi-select would be really really useful, as designed initially by Lada Cmolik (multifocal highlighting, http://www.scitepress.org/DigitalLibrary/Link.aspx?doi=10.5220/0005781301460152).

Standard use case: clicking (thus selecting) a "core" component highlights its dependencies, which I would like to add to the selection, then form a group out of this selected set.