* There is now a feature to (de)select all markers. Can be found in menu or
via the hotkey Ctrl A
* The 'Markers' menu is now not drawn when marker set is 'None'
* Some other minor code cleanups
Additional Notes:
* I'm still trying to track down the cause of a bug which means that you can't
select more than one marker (with RMB clicks) at once. For now, just use
borderselect