BackgroundGradient: Add ability to specify complex multi linear/radial gradients

[chrischrischris]

Expected Behaviour

An end user should be able to specify a gradient that contains multiple linear-gradient and radial-gradient.

Actual Behaviour

Only a single gradient can be defined.

Sample Code that illustrates the problem

Trying to use:

background:
linear-gradient(217deg, rgba(255,0,0,.8), rgba(255,0,0,0) 70.71%),
linear-gradient(127deg, rgba(0,255,0,.8), rgba(0,255,0,0) 70.71%),
linear-gradient(336deg, rgba(0,0,255,.8), rgba(0,0,255,0) 70.71%);

by pasting into the the CSS field results in the gradient being changed to:

linear-gradient(217deg, rgba(255, 0, 0, 0.8) 0.0%,rgba(255, 0, 0, 0) 14.1%,rgba(0, 255, 0, 0.8) 28.3%,rgba(0, 255, 0, 0) 42.4%,rgba(0, 0, 255, 0.8) 56.6%,rgba(0, 0, 255, 0) 70.7%)

Implementation:
Since the interactive gradient creator does not support multi-gradients, a new checkbox or field to be added that allows for multi-gradients. In place of the interactive creator, a preview of the gradient will be shown.