Allow configuring legend patch shapes by double-clicking on legend items

[nyalldawson]

Allows users to set custom legend patch shapes for items by entering a WKT string representing the patch geometry

Sponsored by SLYR

[qgis-bot]

@nyalldawson
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
Also, any commit having [needs-doc] or [Needs Documentation] in will see its message pushed to the issue, so please be as verbose as you can.

Thank you!

[nirvn]

Makes for really nice legends when you want to highlight a specific geometry on the canvas:

PR working well, nice job. Can't wait to see the cool things people will make out of this one.

[roya0045]

Is there a maximum extent or will it resize to encompass all points?

[nyalldawson]

@roya0045 the shape is resized to fit the desired legend symbol size

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#5326
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!