[WIP] [Draft] Touch selections

*This is work-in-progress, just putting this up for anyone who is curious and wants to give it a whirl.*

`EventManager` now exposes a new custom event called `longpress`. Internally, this is a `CompoundEvent`, which is basically what we'd call an event/gesture, not part of any browser APIs, but what can be represented as a logical consequence of several pre-existing events working together.

#### Features:
- [x] Select a word on long-press.
- [ ] Draggable handles at a selection's endpoints.