Added Gradient to Custom palette

[kanurag94]

Type: Enhancement
Part of #165

What's new?
This PR will add feature for users to know beforehand the color palette going to be generated.

Where to see changes?
In GUI menu, Fractal->Palette->Custom Palette

[kovzol]

I like this change!
I think that the current behavior should be however simplified by removing the upper part of the window. Currently the new values seem to be some random numbers. What if the old values are just copied to these input boxes (below)? The user can still press Cancel if the color schema is not the expected one.

[kanurag94]

Currently the new values seem to be some random numbers.

They are the last fed values/ current values already.

The user can still press Cancel if the color schema is not the expected one.

I agree, though I thought if a user wanted to see the difference most importantly in shift values, they would be interested in reading minute changes from previous shift values.