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).
If needed, a convenience function `multiSampleValueToString(int value)` could be added .. then it would become:
```
foreach(int value, QString<int>() << 0 << -1 << 2 << 4 << 8 << 16)
  {
  this->FSAAComboBox->addItem(this->multiSampleValueToString(value), value);
  }
```