Transform: option to use incremental snapping

Transform: option to use incremental snapping

This resolves this issue where users would enable a snapping mode
besides incremental (vertex for eg), then notice strange behavior w/
rotate and scale.
While this ability can be useful, it's quite an obscure use case.

Now changing snap-modes keeps rotate and scale using incremental snap,
with the option for these modes to be affected by other snapping modes.

D4022 by @kioku w/ own minor edits.