Data Explorer refactoring

[klytje]

I think the Data Explorer (file manager) should be refactored to better support passing files back and forth between windows.

First, all file IO should be centralized to the file manager, which should be able to read all types of files. Perspectives may then inform the manager of what file types they support, which the manager may use to "grey out" invalid files depending on which perspective is currently in focus.

Next, perspectives should be downgraded to regular windows, in line with the other calculation tools. This will allow us to decouple the Send to functionality from perspectives, and instead allow it to send files to all window types.

This will be useful in the following situations:

1. We can completely remove file IO operations from the GSC window. Instead, users may load the data in the file manager, open the GSC, and send the relevant files to the window using the Send to button. This, I feel, is a more natural workflow, and matches the current workflow for loading files in e.g. the Fitting perspective.
2. In shape2sas plugin models #3544, a new plugin model is generated based on the user-defined shape. After this shape has been fitted to a data curve, the result could be stored in the File Manager, and the user would then be able to send it back to the Shape2SAS window to easily visualize the optimized shape & parameters (only fitted parameters are visible in the Fit Panel; there is currently no way to view the dependent (constrained) parameters).
3. I intend to add SAXS calculations to the Fitting perspective in Support for SAXS fitting #3194, but work on this has stalled because there is no good way of loading multiple files into the perspective. This refactoring would allow the user to load, mark, and send both files to the perspective in a natural manner.