Download structures loaded via URL before parsing

If a 3D structure file is loaded via URL, then the actual file should be downloaded as a local temp file and used to replace the URL reference before parsing the file.

In earlier versions of Jalview, rather than storing the structure data retrieved via the URL, a hard reference would be recorded in the Jalview datamodel, that would be saved/restored in the Jalview Project. Importing the project and viewing the structure then required retrieving the data again. This would fail if the network environment had changed or the URL was no longer valid. Instead, the data should be downloaded and saved within the project file.