ENH: Allow for specifying number of samples

In concert with https://github.com/commontk/CTK/pull/483, this enables specifying the number of multisamples (e.g., for systems where auto-detection fails).
I would actually prefer to keep this as `QString`. This way any other function that wants to access or set this property (in this module or elsewhere) will do so by using the clearer string interface, and the functionality for converting to said strings into the "CTK format" (-1, 0, 2, 4, 8, 16 integers) is encapsulated within this module. Does that make sense?