Refactor libwindowmanager and libhomescreen out of WAM

Extract those two libraries out of WAM.

Right now, WAM copies and adapts code available in run-xdg for setting up application launch and window management through libwindowmanager and libhomescreen. A common helper library should be implemented that will be reused from run-xdg and WAM with this implementation. This library should make life easy for people willing to port their applications to AGL.

On top of that, Qt-like APIs should wrap the new library, and be added to QtAGLExtras module.