Deactivate export buttons when process does not contain images

[solth]

Fixes #3292

[matthias-ronge]

First issue: Each time the user interacts with the user interface, for each process that is displayed in the list, it is checked eight times on the file system to determine whether the process contains images.

In the Restore View phase:

• to determine the style for the commandLink exportMets
• to determine the disabled status for the commandLink exportMets
• to determine the style for the commandLink exportDms
• to determine the disabled status for the commandLink exportDms

Four more times in the Render Response phase. Caching should be implemented here.

Second issue: The way checking for images is error-prone, as it doesn’t handle all special cases. I would suggest the following.

public boolean hasImages(int processId) throws DAOException {
    Process process = ServiceManager.getProcessService().getById(processId);
    Folder generatorSource = process.getProject().getGeneratorSource();
    if(Objects.isNull(generatorSource)) {
        return false;
    }
    Subfolder sourceFolder = new Subfolder(process, generatorSource);
    return !sourceFolder.listContents().isEmpty();
}

BTW, I am unsure if this method should be in FileService or ProcessService.

[matthias-ronge]

At the moment, we do access File Service using the Service Loader. However, I prefer static access where possible. See #3327.

[solth]

So I take it you agree with hasImages remaining static?

[matthias-ronge]

@Kathrin-Huber should decide

[matthias-ronge]

This seems to cover all requirements.