Use the new molecule signal instead of clearing

Full or partial clearing of the molecule seems hackish, and is likely
to have unintended side-effects. Intead, have QTAIM emit a signal to
let the main window know a new molecule is ready. This then allows other
variables to be cleared correctly, and prevents implicitly overwriting
the active molecule.

Change-Id: Ie4968dbe3733872688bc20130e9c3778909b7e5f
Merge topic 'qtaim-new-mol-signal' into master

b3f865f1 Use the new molecule signal instead of clearing