Fix excessive text elision workbench data view

[martyngigg]

Description of work.

This work addresses the issues with excessive text elision in the matrixworkspace data view after upgrading to Qt 5.12. See the screenshot in the linked issue for a comparison where the old version is on Qt 5.9.

It is not clear that this is a bug in Qt, just a change in behaviour. Here we introduce a delegate to control the amount of elision of the string that occurs.

To test:

• Open workbench
• Load any data file
• Show the data and most digits after the decimal point should be shown. Widen the column to show everything.
• Also check the background colours are respected. Run MaskDetectors and mask out several rows. Open the data view again and see the masked rows have a grey background color
• Check the monitor background is colored yellow. Run LoadEmptyInstrument with InstrumentName=IRIS. The first two rows in the data view should have a yellow background

Fixes #34101

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