Advanced Editing support for Gradients

The Rectangle elment can have a gradient. That gradient can only be linear, and either vertical or horizontal.
The color interpolation is linear through RGB-space and unlike design tools such as Photoshop does not support non-linear (gamma-curve based) interpolation which appear more pleasant to the eye.

This task is about researching what can be done to improve Gradients in Qt Quick and how gradient editong can be improved in the Qt Quick Designer.