Add rounded corner support for rect in kdenlivetitle

[chocolateimage]

MR on the Kdenlive repo: https://invent.kde.org/multimedia/kdenlive/-/merge_requests/838

• Uses a new property called cornerRadius which is set on save from the titler in Kdenlive.
• Adds a new class KdenliveGraphicsRect for drawing the rect (couldn't figure out how to make QGraphicsRectItem rounded otherwise).

[ddennedy]

@j-b-m or @jlskuz needs to approve kdenlivetitle PRs

[j-b-m]

Thanks for this MR, Looks good. I think we should also raise the Kdenlive Titler producer version here, so that we can enabled/disable the rounded corner option in Kdenlive depending on the producer's version.

[chocolateimage]

Thanks for this MR, Looks good. I think we should also raise the Kdenlive Titler producer version here, so that we can enabled/disable the rounded corner option in Kdenlive depending on the producer's version.

I have now changed the version in 9aa477c