Loading from AddWorkspaceDialog freezes the GUI

[robertapplin]

Describe the bug
When loading a file from the AddWorkspaceDialog's in the Inelastic interfaces, the GUI will freeze while loading takes place. We don't want this to happen. Instead, we want the loading to be performed in a background thread so that the GUI remains responsive. We probably also want to disable the "Add" button on the dialog until it is finished loading. Perhaps changing the "Add" button text to "Loading..." would also make it clear that the data is loading during this time.

In previous situations, I have created a "Manager" class to handle the background thread execution for algorithms. As an examaple, for the ALFView interface, see this class https://github.com/mantidproject/mantid/blob/main/qt/scientific_interfaces/Direct/ALFAlgorithmManager.cpp

To Reproduce

For example

1. Open Inelastic QENS Fitting interface

2. Go to F(Q) fit tab

3. Click Add Workspace

4. Browse to the following workspace
   irs26176_graphite002_red_Conv_seq_2L_0-50__Result.zip

5. The GUI will freeze while loading the workspace

6. When finished loading, the GUI will unfreeze

Expected behavior
The loading should be done in the background thread so the GUI remains responsive. The Add button should be disabled while loading, and the text changed to something like "Loading..."

Platform/Version (please complete the following information):

• OS: All