iMovie installation stuck at 100% on Mac, unable to shut down or restart

Yesterday I started an update for iMovie in AppStore and now for a second day it says "Installing: 100% Complete". If I try to shut down my Mac or restart it appears an window from Install in Progress which says that iMovie is being installed.

So what should I do? On one hand I have a never ending update for iMovie and on the other I can't shut down my computer!

[Edited by Moderator]

Original Title: Install in Progress