[Kernel] Fix small bugs in fx diagram editor
There are a few minor bugs in the new editor:
- After dragging a connection, the start location of the gesture is not passed to the link function. The default link controller always passes null.
- In SVG export the text is a little too high (SVG expects coordinates for the text baseline rather then the upper left corner).
- Components cannot be resized to the minimum size (as defined in DefaultLayoutConstants and used by the old editor).
- When dragging the port on a component, the port isn’t under the mouse but slightly off to the side.
- The border location calculation on ellipses should return a point with the correct angle from the center.
(from redmine: issue id 3924, created on 2020-02-03, closed on 2020-03-16)