Multiple selections

See Issue #778. Not everything is perfect yet, but it works quite well.
Todo:
- [ ] testing on a mac
- [x] block selecting across the edges doesn't scroll the editor
- [x] multiple block selections (alt+drag, release, ctrl+alt+drag)
- [x] smarter copy & paste
- [ ] keyboard shortcuts
- [x] autocomplete
- [ ] clearer styling of adjacent selections (with a border or rounded corners), although showCursorWhenSelecting also helps
- [ ] the new code in adjustPos is probably wrong (the second branch, which adjacent selections need, breaks the vim and emacs modes, but I don't know vim or emacs so I'm not sure when or why. with a single selection everything is fine though)