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.