Dragging workspace to desktop doesn't work with Qt

[hcferguson]

The manual says "Tabs can be freely dragged between workspaces, or out onto the desktop, forming a new, detached workspace.” …tried dragging the Thumbs tab to the desktop with no success. On the phone @ejeschke said this works with gtk, but not Qt.

[ejeschke]

Currently this is due to the difficulty of achieving this easily in Qt widgets. I believe there is a way, but it's not clear from any documentation I can find.