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).
By connecting the signal `currentIndexChanged(int index)`, the code above would then be simplified to:
```
ctkVTKAbstractView::setNumMultiSamples(d->FSAAComboBox->itemData().toInt())
```