Re #7442. Use MWRunFiles widget in DGS reduction GUI.

It has to be swapped in at runtime because:
(a) pyuic doesn't know anything about MWRunFiles, &
(b) there's a desire to have these GUIs runnable even from outside MantidPlot
Re #7442. Add ability to set minimum width on the label.

Also expose this new method via python.
Re #7442. Apply minimum width to label in MWRunFiles widget.

This restores the alignment of the text edit boxes.
Re #7442. Fully hook up file finding signals.

Validity of the initial content is now checked and the validation is also
hooked into the reduction gui's 'yellow background' indicators.

Exposed further methods of MWRunFiles to python as required.
Merge remote-tracking branch 'origin/feature/7442_mwrunfiles_in_dgs_reduction_gui'