[KFilePlacesView] Use asynchronous KIO::FileSystemFreeSpaceJob

This ensure hovering a broken mount will not freeze the UI.
I already optimized it in D11088 to avoid doing blocking calls for when the bar isn't shown but this patch makes it fully async now.
Moreover, refresh only every 60 seconds (it's what Dolphin as well as Plasma's free space warning do),
currently it would refresh every frame

Differential Revision: https://phabricator.kde.org/D16311