Blender Git Loki

Fix T63994: Node Editor: Move All Selected Nodes when dragging.

Left-click select broke that behavior, since it puts both action and
select buttons on the same physical mouse button...

To support this behavior again, we have to split selection process in
two steps, hence make it modal... While I remain rather skeptical about
that global design decision, and complexity it adds to many UI/UX areas,
this solution ends up being OK-ish I think.

Thanks to @brecht for some final tweaks on the patch.