Add option: don't delete unused gradients

Hi All,

Saving gradients within a session is now more difficult than before.

In the past, new gradients were still available for use, even if no objects had the gradients applied.

Steps to reproduce:

1. Apply a gradient to an object in the workspace.
2. Apply a second gradient to a second object in the workspace.
3. Observe in the XML editor, that the attribute [inkscape:collect always] is applied to each parent gradient.
4. Delete the attribute from one gradient, via the xml editor.
5. Delete both objects from the workspace.

What happened?

The gradient having the attribute [inkscape:collect always] is no longer available for use on other objects, whereas the gradient without the attribute is still available for use.

What should have happened?

Creating gradients should not apply the attribute to the parent gradients, just the child (linked) gradients.

• Note that this is the expected behavior, as this is how the legacy gradient editor works.
• Note that any unused gradients are still removed entirely by cleaning the document.

...

Inkscape Version and Operating System:

• Inkscape Version: Inkscape 0.92.2 (5c3e80d, 2017-08-06)
• Operating System: Win
• Operating System version: 8.1

Here's a gif screen capture example:

Thanks for your consideration, TD