Custom position marker rotation
completed by: marthd
mentors: Dennis Nienhüser
When the user configures Marble to show a custom position marker image, it should be rotated according to the GPS direction. This makes it possible to use e.g. arrow images and have them point correctly in the driving/walking direction.
It is ok to assume the position marker image provided by the user points to a certain direction (e.g. north), but the config dialog should tell the user about it (using e.g. a label that says so).