Project Explorer allows to import folder as project with the context
menu action. The problem here is that if you have lots of folders to
import, you are forced to import them one-by-one. This PR proposes a
change which makes this menu entry available and usable on multiple
folder selection. Entry will still be unavailable, if any of the
selected items is not a folder with project description file in it.
Corresponding test checking item availability was also added.

Fixes eclipse-platform/eclipse.platform#226

Signed-off-by: Nikifor Fedorov <nikifor.fedorov@arsysop.ru>