editor.diagram: add support for grid snapping

[alexanderpann]

Diagrams can now show a grid that is used for snapping elements to it. It can be configured (visibility, grid snapping, grid color/size/style) in the editor definition of the diagram itself:

The elements sometimes don't snap to the grid itself but that seems to be an issue in the library itself. Taken from https://github.com/jgraph/mxgraph/blob/ff141aab158417bd866e2dfebd06c61d40773cd2/java/src/com/mxgraph/swing/handler/mxGraphHandler.java#L1105:

Drag image size depends on the initial position and may sometimes
not align with the grid when dragging. This is because the rounding of the width
and height at the initial position may be different than that at the current
position as the left and bottom side of the shape must align to the grid lines.
Only fix is a full repaint of the drag cells at each new mouse location.

[HeikoBecker]

Thanks for implementing this feature. Looks great!

LGTM.