Implement automatic load

[fedorov]

Open questions:

• how much it makes sense to load
• what to do if selection includes pathology images
• what if there are series that do not have appropriate plugins

Need to discuss with @rkikinis.

[fedorov]

Need to test current behavior, see what is happening if a modality that does not have a plugin is being requested to load. Investigate if we can use existing hard-coded mapping between modalities and extensions, and offer user to install the missing plugin. We cannot add all of the extensions as dependencies, since we do not want failure of any such extension to break our extension.

If we want to offer install of extension to the user, need to discuss if this should be done in this extension, or added as a core app feature.