Reworked to open/close logic to do some speed-ups:

* We no longer close the camera immediately after every request.
* We close the camera after:
- There are no pending requests for this slave AND
- The idle timeout of 30 seconds has been reached.
- Another slave marks that it needs the camera.

A lock file (locate("tmp","kamera")) is used for marking "give up camera".

Did some fixes (gp_file_free -> gp_file_unref) to avoid crashes.

Also did some clean-ups in the header file.