Improve progress notification for slow calculations

[richardh]

When doing long slow calculations, it is irritating to keep having to open the console to try to work out whether sasview has finished or not, or indeed whether I have really have clicked on "fit" or "compute".

There may be several things that could be done such as:

Grey out, change colour, or disable the "fit" and "compute" buttons after they are clicked.

Change the previous message at bottom left of screen to "calculating"

Preface the "Computation completed!" message at bottom left with time in "hh:mm:ss"