Description of work

This PR fixes a warning that can appear whenever data is being loaded from dialogs in all tabs of QENS fitting interface. Even if the data is taking a long time to load, it is still possible to click on the Add button of the dialog window, which will cause the interface to try to act on the data while is being loaded on the ADS, thus creating warnings or crashes.
The solution I have adopted to fix this undesired behaviour from the dialogs is to emit a signal from the Data Selector widget when it autoloads the data. This signal can then by received by the dialog interface to disable the Add button (and change its text) until the loading of the data is finished.
However, there are still problems with the dialogs:

• If the dialog window is closed while the data is being loaded, this will create a crash in Mantid (this is not happening due to the changes on this PR, it's a current issue with the dialogs).
• Interface can still freeze while the data is being loaded if trying to interact with it.

A more fitting solution would be to implement a manager class for the loading algorithm in a different thread with which we can communicate (#37184). There are plans to implement that solution for Mantid 6.11. But I still think emitting a signal from the selector widget when it is auto-loading may be a good inclusion. And disabling the button helps the users understand what is happening with the data.

Summary of work

Fixes #37185 .

Further detail of work

To test:

• Follow instructions on F(Q) Fit warnings after adding data multiple times #37185. You shouldn't be able to click on the Add button until the data is loaded.

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