Further separate icon from text of buttons in both editor and default themes

[YeldhamDev]

I've noticed that the the icon and text separation in buttons seems a little too modest, making it look like there's almost no separation at all. This PR makes so that the default separation is 4 instead of 2, and it also removes the BottomPanelButton theme variation, as I don't feel like it's needed after this change.

Before:	After:

[Maran23]

A separation of 4 is also the default on other UI libraries like JavaFX. So it seems like a good choice.

[Calinou]

This will change existing projects' theme appearances if they don't override the constants in question. I think it's a good change, but it's unfortunately technically a breaking change 🙁

[YeldhamDev]

@Calinou This has already been talked about in the dev chat, and acceptable for a minor release: https://docs.godotengine.org/en/stable/about/release_policy.html#what-are-the-criteria-for-compatibility-across-engine-versions

[Calinou]

@Calinou This has already been talked about in the dev chat, and acceptable for a minor release: docs.godotengine.org/en/stable/about/release_policy.html#what-are-the-criteria-for-compatibility-across-engine-versions

That particular note is for the default project theme, not user-made themes that happen not to override the h_separation constant.

Either way, the visual difference with this PR is small, except in games with a pixel art appearance that use the viewport stretch mode and a low base resolution. For instance, increasing icon spacing from 2 to 4 pixels on a 340×240 canvas is pretty significant in comparison to performing the same at a 1152×648 base resolution.

[akien-mga]

Seems like a good change to me.

As long as we document it clearly in the "Changes" section of the changelog, should be fine for 4.2.

[YeldhamDev]

@akien-mga Just as I removed the default theme changes... 🥲

[akien-mga]

@akien-mga Just as I removed the default theme changes...

Well I didn't consider it as an option but yes, it could make sense to preserve compat for the default theme and only change the editor theme.

[YeldhamDev]

Well, now I just reverted them back. So I hope we can just go with it...

[YuriSizov]

I think it's fine to change this for a minor release. Better defaults are always good, and the ability to override them is not going anywhere.

Both default and editor themes look fine IMO, nothing stands out:

[akien-mga]

Thanks!