Draft of a new GUI arrangement

This release adds new controls for managing Athena's graphical rendering, and a single unified geometry chooser drop-down. Only relevant tools are offered to the user (PERDIX and METIS when a 2D file is open, TALOS and DAEDALUS when a 3D file is open). We are getting close to being feature-complete for a 1.0 release.

Feedback on the new GUI layout is invited, as layout and presentation decisions are the most important issue right now.