make gradient auto collection optional.

This is my first ever MR to any FOSS project. Thanks to the @jabiertxof @adam.belis for the support and guidance.

Summery

This MR is trying to fix this UX issue: ux#138

• Basically, now we made a inkscape:collect = always as a optional.
• User can now enable/disable the gradient auto collection thought preference dialog. So, that user can preserve the gradients that are not used in the SVG file for the latter use.
• This option will only affects the newly created gradients. Gradients that are already collectable, will be not updated. (I'm working on a extension to make all the available gradients non-collectable or vise-versa to solve the problem. or maybe different approach. will see it latter what UX team will think about it.)