Now we have a better distinction of what is snap to grid and what is
snap to increments.
The code also allows the implementation of mixed snap for these modes.
This replaces header include guards with `#pragma once`.
A couple of include guards are not removed yet (e.g. `__RNA_TYPES_H__`),
because they are used in other places.
This patch has been generated by P1561 followed by `make format`.
Differential Revision: https://developer.blender.org/D8466
Now all options for "snap to" affect the Vert Slide mode.
Reviewed By: campbellbarton
Maniphest Tasks: T66426
Differential Revision: https://developer.blender.org/D3440
Currently, this change does not bring functional changes.
But it is necessary to extend the use of the snap system for gizmos,
since, after a Undo, the `depsgraph` pointed by the `snap_context`
has its memory invalidated.
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D7013
