This would require a fair bit of modification to the code code, plus to the themes to support.
If you’re looking to do it though, for the twig side (to display them) you want to look into all-in-one-event-calendar/public/themes-ai1ec/vortex/twig , specifically at:
filter-menu.twig
categories.twig
tags.twig
This will give you some idea how they’re called from each other, and what you need to do to create the pull-down filter arrangement in a .twig file. There will of course be CSS (and possibly Javascript involvement as well) that you’ll either want to copy or modify as well.
You will also most likely want to look into all-in-one-event-calendar/app/view/calendar/page.php , all-in-one-event-calendar/app/model/taxonomy.php , all-in-one-event-calendar/app/model/filter , and all-in-one-event-calendar/lib/twig/ai1ec-extension.php at a bare minimum.
Note: If you’re really interested in doing that, just beware that 2.0.7 is due to be released shortly, so you might want to wait till that’s been released before you start on your work, as it should fix a number of bugs in the Core code that you might need to otherwise fix.
