KFilePlacesItem: Use Solid to find home mount point
KMountPoints potentially parses MTAB which can be slow. Instead, use Solid which already has a cache of all the devices.
The impact on a typical system is negligible but real-world testing on a system with lots of network shares showed a speed-up of over 80% in this code path.
Signed-off-by: Kiriakos Antoniadis kiriakos.antoniadis@advantest.com