Lazy-load ImageServer from ImageData

[petebankhead]

Rough draft at an alternative approach to address the issue raised in #1488

[carlocastoldi]

This approach is much more elegant than #1488, i like it!
However I feel it doesn't exploit all the possible info that can be gathered without reading the image files. Specifically images' metadata for info such as the number of channels and their names or even as well as pixel calibration.
If available, they're available in the .qpproj and they're already read when opening the project.

Perhaps this could be done by exposing a getCurrentMetadata() method, so that it does not require to pass through calling getServer()?