Tweak Quick Open theming

[MewPurPur]

Addresses some, but not all points in: godotengine/godot-proposals#11558

The things I've fixed are the issues with spacing and sizing.

Changed up the structure of list elements to prevent thumbnails from stretching vertically. Here's how this looked before:

Changed up the structure of grid elements too, to give some space above and to have better spacing. Here's how it looked before:

And there's now no spacing between elements, prevent you from accidentally clicking on the tiny space between.

I've also fixed the ugly hover style, which came from overriding the nonexistent "hover" style instead of "hovered".

Here's how it looks now:

Please tell me if you think the spacing is too tight now.

[Calinou]

Tested locally, it works as expected.

The amount of spacing looks good to me. However, the icons aren't centered in the hovered/selected area:

There are 6 pixels above the icon and 7 pixels below the icon. (The icon is 36 pixels tall.)
I can notice this issue across every item, so it's not specific to some items on the list.

I suggest adding 1 pixel of padding at the top so it's even.

[MewPurPur]

Sure. This is because the vboxcontainer with the text is 37px tall, because text has this tendence of taking more space than what it seems like it should.

I'll fix it.

[MewPurPur]

Doned, bumping up the "anti-separation" of the two labels by 1 more pixel looks good to me even with the maximum ascents/descents in the default font.

[Calinou]

Looks great and even now 🙂

It's still a tad uneven at 200% editor scale (10 pixels above the icon, 11 pixels below) but it's probably unnoticeable on an actual hiDPI display.