Sky plot: NESW marks (fixes #15), code cleanup. Thanks paulle!

Introduce mRotation member (actual rotation, based on azimuth and screen
orientation)
Avoid allocating paths and paints at draw time but use preallocated
instances, listen for azimuth and size changes and update anything
depending on these.

Signed-off-by: mvglasow <michael -at- vonglasow.com>