Cancel Data Load on Load

[JackEAllen]

Desired Behaviour

When loading a dataset into mantid imaging, a progress bar dialog window opens. We should add a cancel button to this dialog to allow users to cancel loading of a dataset part way through loading. Closing the progress bar dialog window does not achieve this. The data will still load in the background.

Current Behaviour

Mantid Imaging does not support canceling a dataset on load.