[KFilePlacesView] Show free space as permanent thin line

This implements the behavior how Dolphin used to do it with a custom painted blue (or red, if space is running low) line.

The KCapacityBar is just too clunky to fit into this tight space and depending on style might exceed the available bounds or otherwise render incorrectly.

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It polls all devices every minute. This is how Dolphin used to do it but I wonder if we should stop the polling if the window is inactive (isn't focussed)? This could lead to inconsistent results if you have two windows in split view side-by-side.

However, I think this is just a small visual element and at least personally I always have a Dolphin open in the background and I wouldn't want it poll all my devices when I'm not really using it.

Before (Breeze)

After (Breeze)

Before (Oxygen)

After (Oxygen)