Fix DesktopSchemeHelper's QML singleton-ness
After some refactoring in c116c00d, the object had lost its ability to be accessed from QML and its singleton-ness.
Fixing that fixes these issues in Folder View:
- Places and Custom location options not working
- Drill-down folder popup not working
BUG: 498783 FIXED-IN: 6.3.0
Edited by Nate Graham