Improve DockContextPopup spacing

[KoBeWi]

I noticed that after adding the new bottom dock slots, the new slots have confusing margin at the top due to not having any tabs by default. So this PR adds a separator, which makes them look more like slots. Also addresses #114360 (comment)

Before	After

HOWEVER 💀
There is a problem with the new size. Ideally the bottom panel should take 3 cells in the grid, but in this PR it takes 4, making the main screen part awkwardly square, which again does not really represent how it looks normally (or maybe it's like that on bigger screens?). This is because the new bottom slots kinda enforce the width to be even (unless we allow them to be asymmetrical :/). You can check the linked comment for comparison.

[YeldhamDev]

Looks fine on my screen. But someone with a smaller one may object.

[Repiteo]

Needs rebase

[kitbdev]

There is a problem with the new size. Ideally the bottom panel should take 3 cells in the grid, but in this PR it takes 4, making the main screen part awkwardly square, which again does not really represent how it looks normally (or maybe it's like that on bigger screens?). This is because the new bottom slots kinda enforce the width to be even (unless we allow them to be asymmetrical :/).

Couldn't it just use Rect2 instead of Rect2i and pass 2.5 for bottom docks? The max_tabs can just be floored.

[KoBeWi]

Using Rect2:

The gap between bottom-most slots is 1 pixel smaller, but otherwise does not look bad I think?

EDIT:
On another look, this one looks more square actually🤦‍♂️
So I guess the PR is fine as is.

[Repiteo]

Thanks!