Add qtAdvancedTableView with multile-row, column-spanning header

This branch derives from an open source project (MIT license) that subclasses QTableView and QHeaderView to extend the header widget so that it can be configured with multiple rows and column spanning. The intended use is for a tabular attribute view to be developed in the future. An example using the new code is included; when the cmake variable SMTK_ENABLE_EXAMPLES is enabled, an application qtAdvancedTableMockup is built in the bin folder; running the example will display a (non-working) example of the planned tabular attribute view.