Clarify filtering by node type and group in the Scene tree dock

[Calinou]

This functionality has been available since 4.0, but was difficult to discover since there was no tooltip and the placeholder didn't mention it.

Preview

To make the most important part of it (t:type) squeeze within the default dock width, I had to be a little creative, but it should be self-explanatory enough:

[KoBeWi]

Filtering is available from the ⋮ menu.

[YuriSizov]

Filtering is available from the ⋮ menu.

I don't think that the hint in the placeholder (and a tooltip) hurts, though, does it?

[KoBeWi]

It was more response to "was difficult to discover since there was no tooltip and the placeholder didn't mention it.".
I think tooltip wouldn't hurt. Not sure about placeholder, as it can't fit all filter options.

[Calinou]

I've changed the placeholder to also mention g:group. At the default width, it's conveniently hidden, but you'll see it if you resize the dock's width:

[akien-mga]

Thanks!