[client] queueing installation

[fussel]

TL;DR Let us queue up installations of our games


Some of us have increasingly less time to play games and want to do other stuff than waiting, watching a progress bar fill up while "preparing files".

Therefore I'd like to see the install mechanics a little revamped, where instead of preparing files and then starting the download in the manager (thus blocking subsequent install orders until preperation is completed), steam puts the operation in the dl manager right away and processes non prepared games in a serial fashion, one by one.