Draw 2 Rectangles below devices to represent a disk capacity bar

FEATURE: 315405

This is a continuation of https://phabricator.kde.org/D21312, this new MR includes:

• It will only show on devices. It no longer show under mounted folders like Root in the user's favorite places.
  isDevice = !data().value("udi").toString().isEmpty()
• Someone pointed out that the KIO FilePicker now uses KIO::FileSystemFreeSpaceJob, so I've refactored the patch to use that API.
  • https://github.com/KDE/kio/commit/933887dc334f3498505af7a86d25db7faae91019
  • https://github.com/KDE/kio/blame/master/src/filewidgets/kfileplacesview.cpp#L229

TODO:

• Confirm performance impact.
• Check if an unmounted drive does not shows a capacity bar at startup.
• Check if an unmounted drive shows a capacity bar after mounting.