libdeng2|Loop: Added static accessor for the app's Loop
Refactor|libdeng2: Improved Observers template with QSet and thread-safety

Thanks to the implicit sharing of QSet instances, Observers<> is now
protected against modifications to the set while it is being iterated.
Also, Observers is now ReadWriteLockable and automatically locks itself
as appropriate for threaded access.
Refactor|libdeng2: Notifications from a threaded Bank

It is not allowed to start a QTimer belonging to another thread. Instead,
the Loop iteration audience is now used for notifying users of a Bank
when background tasks have completed.
Tests|glsandbox: Fixes for Windows

Install the testpic image and make sure the OpenGL context is current when
uploading a GL image.