Fix error when editing gui with taurus newgui

Fix an error when trying to edit an existent gui with the taurus newgui command.
The problem happened when sardana was not installed. In this case, the sardana-specific wizard pages are skipped, but when loading a previously created gui, it tried to load de xml of these panels anyways.
This MR closes #1200 (closed) and #1333 (closed).

Note: if a GUI was created in an environment with sardana and is edited in an environment without sardana, the sardana configuration will be lost.