Provide pressed feedback for ResultView

Use "down" instead of "pressed" (which is read-only) for indicating pressed down state, and make the highlight show feedback for when the current item is "down".

Also use highlightedTextColor accordingly.

Since the ListView has its own highlight, the ItemDelegate's will not be shown.

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@alex @ratijas

So it doesn’t technically need libplasma!991 (merged) to work

Not entirely sure what the history behind the tinting was (I think there was a problem with just using opacity), so I tint it a bit less when pressed otherwise the color becomes super hard to read.

While at it used Qt.alpha and split it into a dedicated property so it can be reused

Also switch icon to selected state