delete dependency checks

Covers most of https://github.com/KSHSK/WAVED/issues/183.
* "Don't allow a DataSet to be deleted if used by a QueryAction" - cannot be done until QueryAction is implemented.
* "Don't allow a DataSet to be unbound from a Widget if that Widget uses the data" - the framework is there, but changes will be needed later. I implemented this by adding `WidgetViewModel.prototype.usesDataSet(dataSet)`. `WidgetViewModel` always returns false for that function, since most widgets do not use data in any way. This means that once `USMapViewModel` and `GraphViewModel` are implemented fully, those checks will need to be added there.

The above issue should not be closed even when this PR is merged in since those two bullets will still need to be resolved.