Fix read-only EditorSpinSlider display

[MewPurPur]

Prevents these things from being displayed in read-only spinsliders.

Advances godotengine/godot-proposals#6988 a little.

[RedMser]

Without the grabber showing, there is no indication of the slider's current value in relation to its minimum/maximum values. So if you e.g. see a 0 you don't know if the slider also accepts negative values, or if it's on the minimum already.
Would it maybe possible to reduce the opacity instead of completely hiding it, so it's clear you can't edit it?
Then again I can't think of many cases where a slider is read-only in the UI, so maybe it's not important.

[MewPurPur]

@RedMser It's only the grabber that doesn't show, the rest of the slider does.

[MewPurPur]

Poke poke. Should be an easy merge, nothing can really go wrong here.

[akien-mga]

I would suggest rebasing for good measure, it's been 6 months.

[akien-mga]

Thanks!