Similar to Qt::GlobalColor, the presets allow the user to create
brushes based on predefined gradients, quickly getting pretty pixels
on screen.
The presets are based on the linear gradients from WebGradients, a
free collection of gradients, hosted at https://webgradients.com/.
The few radial and blended gradient presets have been excluded.
Change-Id: I1ce8f2210a6045c9edb8829ab3eddcc313549127
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
