Change UIs to inherit from QWidget instead of QDialog so they can be used in "docked" style

[nxkb]

Some of the dialogs have a significant amount of machinery in the dialog itself, which you lose if you want to use the model / view in a docked widget. We should be able to split any dialog specific functionality into its own class that simply wraps the widget. Then the client can use whichever one they need and not lose out on any of the good stuff.

In particular, this should be done for: UsdOutliner, EditTargetDialog
The LayerTextEditorDialog looks like it already wraps the LayerTextEditor.

[nrusch]

This should be addressed by bdef955 (currently on dev)