deprecate methods lbegin / lend / leafbegin / leafend

As decided on the developer meeting in Aachen, the mentioned methods are
deprecated on the Grid interface class and the GridDefaultImplementation by
this patch.

Note: All grids that implement these methods by forwarding them to their grid
views can simply use the default implementation to inherit the
deprecation. Grids that implement their grid views through the mentioned
methods on the grid (e.g., by using the DefaultGridView) can currently neither
remove them (would generate an infinite loop) nor deprecate them (would
generate deprecation warnings even when used through the grid view).