Add multiple function screens

[ByteHamster]

trafficstars

Currently, we only have the basic calculator. It would be nice if swiping left and right revealed constants and more advanced functions.

Suggestion on how to do this:

• Make each individual screen a fragment that is displayed in a ViewPager
• Pressing a button sends an EventBus event
• The main fragment registers to the events and adds them to the text box

Doing this with individual fragments and events would decouple function screens and main calculator screen. Maybe that makes it easier to design/show/hide different screens.