QtVersion: Allow for variables in Qt version names

* Supported variables are %{Qt:version}, %{Qt:type} and %{Qt:mkspec}
* Qt version display names need no longer be unique. This simplifies
  the code a bit and makes the Qt versions behave like the Kits do now.
* The default Qt version name now contains %{Qt:version}. The method
  to find the default version name no longer takes a Qt version string.

Change-Id: Ibca1e3daffe5a81f3c158e8bbc1779d033b03872
Reviewed-by: Daniel Teske <daniel.teske@digia.com>