Ability to multi-select blocks using visual markers

The problem
Multi-selection of blocks through visual markers is not possible.

The solution
To have the ability to multi-select blocks the same way we multi-select objects using visual markers.

Ctrl+click each block, but I think that using these visual markers can be useful for people not used to keyboard shortcuts. Besides there is currently a bug with the Ctrl+click for multi-selection that I’ll quickly publish.