DataViews: Remove add filter button if no available filters to add

[ntsekouras]

What?

Fixes: #64689

This PR removes add filter button if no available filters to add. @jameskoster do you know if there is a good reason to keep it disabled as is right now?

Testing Instructions

1. In site editor go to list that uses DataViews, like the pages
2. Add all the available filters
3. Observe that the add filter button is not shown

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[oandregal]

I've commented in the original issue because I didn't realize there was a PR for it. Copy/Pasting my comment there:

The Add filter link should only show when there is another filter to add.

I agree with this. If there's one filter, there's never an opportunity for the "add filter" to become enabled.

When there are multiple filters and they've all been added, the Add filter link still shows

This case is different, though.

You cannot add more filters when you've added all of them, but the button becomes active as soon as you remove one. In this scenario the disabled/enabled states are more representative, IMO. It'll be a confusing experience to having buttons coming in/out on the screen.

Additionally, we need to pay attention to the keyboard affordances: notice how the "add filter" button gains focus when the last filter is removed:

Gravacao.do.ecra.2024-08-21.as.22.01.18.mov

[ntsekouras]

Additionally, we need to pay attention to the keyboard affordances: notice how the "add filter" button gains focus when the last filter is removed:

You right @oandregal, I had added that a while ago and forgot about it. Sorry..

I'll close this PR for now in order to have the discussion at the main issue first.