Keep track of which plugins are needed/loaded when saving session

[astrofrog]

As mentioned in #682, we should keep track of loaded plugins when saving a session, since some things that are saved may depend on paths to classes in plugins.

What I suggest we do is that if a user then loads a session but has some plugins missing, we show a dialog on the following lines:

The following glue plugins are missing and may be required by the
session file:

    * glue-exp
    * glue-3d-viewer

Do you want to try and load the session anyway?

                                           [ No ]     [ Yes ]

We could also be smarter and try and figure out which plugins are actually needed for the session to load properly, by inspecting all the paths included in the session file.