Move ::seek from Cursor to Controller in mouse.js

Fun fact: Cursor::seek was only ever called by mouse.js (as opposed to
.seek() on Node's which is called here and by the up/down stuff, and
recursively). Now it gets to be in the same file as its call sites!
Move writeLatex from cursor.js to latex.js

The call to writeLatex in the API actually gets cleaner!

The call in LatexFragment::createLeftOf instead has it inlined, which is
unfortunate, but really just says the low-level API needs to be simpler,
I think; any Node's .createLeftOf calling a high-level method like
writeLatex is still a layer violation.

Also simplify FIXME comment in paste event handler.
Move ::notify from keystroke.js to controller.js

I think it's a core Controller method that's called all over (although
for now, onNotify() is only called in keystroke.js).
Merge pull request #236 from mathquill/refactor.notify-service

Refactor: notify as a core Controller service