Implement first prototype of event system

[elkowar]

After lots of discussions, more or less this is what we came up with:

Input handling should have one centralized input mapper that re-emits events (Semantic and function calls, I guess?) that registers itself, thus we'll (at least for now) also not need to care as much about priorities in the event system itself (although that will still matter in the future). The input mapper may be expressible as a stack of keymaps, but that'll need to be further discussed / analyzed, I think