Merge lp://staging/~gary-lasker/software-center/fix-lp969907-for-5.2 into lp://staging/software-center/5.2

This branch should fix the crasher in bug 969907. Unfortunately, I could not find a way to reproduce this crash, but it has been consistently appearing on errors.ubuntu.com so we need to fix this. Basically, the fix consists in checking for a null model on the call to _app_activated_cb(), and returning if we don't have a model. There should be no unwanted side effect for this fix in the case of the error because we can't view the details for an application if we don't have a model available.

I also made a change earlier in the chain, in the _on_key_release_event() method (which eventually calls self._init_activated and finally results in the callback being called). In this there was a direct call to this.get_model(), which is not valid as it does not check for Gtk.TreeModelFilter as is done in the appmodel property. Instead, I just use the model passed in with the event.

I also did a small amount of cleanup to the code in _app_activated_cb(). There is a lot more cleanup that could and should be done in general in this class, but this branch is targeted for an SRU and so makes minimal changes.

Testing should be done in the various listviews, that is, to use both the "More Info" and the "Install/Remove" buttons in the listviews for both the "All Software" and "Installed" views (the latter tests the treeview case).

Thanks for your review!