[bugfix] fix FS#1390

The addition of the leafGridView / levelGridView methods to the
GridDefaultImplementation caused test-geogrid to fail (see FS#1390).

The problem results from the fact that GeometryGrid does not use the
DefaultGridView but implements its own grid view (as do some other
grids). This patch fixes the problem by making leafGridView /
levelGridView the interface method (i.e., the method implemented by the
grid implementations) and forwards GridDefaultImplementation::leafView /
levelView to asImp().leafView / levelView. The old methods on the
GridDefaultImplementation can now be deprecated properly, letting all
implementations inherit the deprecation.

Note: All grid implementations not using the DefaultGridView must rename
their implementations from leafView / levelView to leafGridView /
levelGridView, now.