Added some useful additional features

Some useful features added to iridium:

1. Added biomodels search capability
2. X-axis now reflects the selected x axis item in time course
3. Added a new syntax that can be used inside an antimony comment to specify how the simulation experiment was done
4. Exports SED-ML of a given experiment
5. Exports a Combine archive of model and simulation experiment
6. Added a help panel in the output section for antimony syntax and the new metadata syntax.
7. Cosmetic: scroll bar appears in time course simulation if window height is reduced
8. Added recent files in File menu
9. Now has proper Save and Save As functionality
10. Saves window position, size and panel positions to preference file so that the same configuration can be restored in a new session.
11. Fixed font rendering of markdown help files.