Generalize GetMeshMTime to vtkDataset

Currently, only vtkPolyData and vtkUnstructuredGrid support the "Mesh Modification Time" feature through their method GetMeshMTime. It is useful to avoid some unnecessary recomputation in cases where the geometry of a dataset doesn't change between updates, but the point/cell data potentially do.

With this MR we move the abstract logic up to vtkDataSet to allow more filters and datasets to take advantage of the feature. The default behavior is to simply return the MTime, so in the worst case any modification is considered to change the mesh.