Qt tabbed ListEditor styles

Second attempt at the PR.  This PR adds a minimal set of support for the `dock_style` trait of tabbed `ListEditors` under Qt. In particular:

* if the dock style is set to `tab` then document-style tabs are used (which is most noticeable on OS X)
* if the dock style is set to `vertical` then the tabs are rendered with the `West` Qt `QTabPosition`