What does it do?

Adds base grid method to differentiate the intent of the category URL param, between being the value of a grid filter and that of the target category for a new Element (when choosing Create a [Element] Here via the contextual menu), and applies the appropriate value for each context.

Why is it needed?

The Template Access grid initially is empty when a new Element is created in the root (i.e., it has no category), because the value of the category param is 'undefined'.

How to test

1. Create new Elements of each type to verify normal operation.
2. Pay specific attention to the Template and TV Elements, where each has an assignment grid (Template Variables and Template Access tabs respectively). Verify these grids show as expected and are unfiltered when creating a new Element and when editing an existing one.
3. On an existing Template and TV, go to the respective assignment grid tab and use its category filter. Copy and paste the resulting URL (in the browser URL bar) and copy that into a new browser tab. Verify that the view in this new tab is the same as the previous tab where you applied the filter. (This is to ensure the somewhat new feature of grid filtering persistence via the URL still operates correctly.)

Related issue(s)/PR(s)

Resolves #16677