Library for painting graphics items

[Marianne Stecklina]

I want to implement an editor, where the user can draw lines, polygons etc. by clicking on a graphics scene. It should for example be possible to grab a corner of a polygon to change its shape.

I know how to do this from scratch by overwriting the scene's mouseevent functions. What I would like to ask: Does anyone know a library that implements all these standard painting editor functionalities to save time?