add category column to stitches pane in UI

[Neon22]

If the category was shown in the stitches pane in the main window then we could sort filtered stitches by category.
This would allow users to bring these stitches to the top of the list.
This would be super useful when we have many more stitches.

The file to edit is mainwindow.ui
The section in the file is labelled:
<widget class="QDockWidget" name="allStitchesDock">

Under here we currently have widget <widget class="QTreeView" name="allStitches">
In addition to the stitch name and icon, please also add category.

(Because we can add categories using the hidden xml import mechanism in the configuration file)

[Neon22]

This is IMHO the first thing to fix if a new release is considered.
Its got the greatest effect - max user frustration (atm) is filtering through so many stitches.
Even more stitches now with replacement sets:

• https://github.com/Neon22/Crochet-Charts-Stitch-Sets/tree/master/Updating%20Crochet%20Charts%20v1.2

Has advantage of not requiring any code changes but does need a recompiile to make a release for Windows and mac.