Add plugin system to the camera application

[dark-eye]

As a result of the discussion at #8 and the request to add #2 it seems that the camera app needs some easy system to add image alteration/processing to the camera application

[UKPhil]

Hi dark-eye, I am just trying to find out the status of the photo date & time stamp. I thought this was fixed and therefore closed, but it does not seem to be the case.