PropertyWidgets: move to MantidWidgets

[mantid-builder]

Original Reporter: @jzikovsky

Original Owner: @jzikovsky

From Martyn:

I was just looking at the stuff you've done with property widgets. It looks good but I wonder if the widgets themselves could be moved to the MantidWidgets lib instead? The interface can stay in API but I think it would better if the widgets themselves lived with the MantidWidgets code.

Also, do you think it would be possible to use these with the Qt designer? It's not quite necessary at the moment but would be good for reusing in custom interfaces.

The other small thing was that there is already a widget for finding files, the rather poorly named MWRunFiles widget. This is currently exposed out the designer for people to drop on the UI forms. I don't know how much of the code overlaps with the FilePropertyWidget but would you mind checking whether some of it could be merged in some way?

[mantid-builder]

This issue was originally trac ticket 4847