Simplify mousemove vs docmousemove handlers

Instead of the arcane `delete e.target`, mousemoves passes in an
optional target that docmousemove omits.

Also simplify and clarify comments.
Fix #312, seek ignoring mousemove target
\MathQuillMathField{}'s listed in .innerFields

Static math instances now have a .innerFields list of the
\MathQuillMathField{}'s that are inside.

How: whenever the LaTeX is set (including on initialization), do a
post-order traversal running .registerInnerField() with an array to
register inner fields on.
Add optional arg to \MathQuillMathField[name]{contents}

See test case
Move .initExtractContents and .latex into .initRoot #simplify
Refactor .init{Root,Events} as .initRootAndEvents

...on MathQuill.EditableField
Inline .initExtractContents into .initRoot #refactor
Rearrange AbstractMathQuill::initRoot #reorg
Merge branch 'dev' into feature.innerFields

Conflicts:
	src/publicapi.js
	- feature.innerFields refactored MathQuill.{Math,Text}Field in
	  src/publicapi.js
	- #319 on dev moved MathQuill.{Math,Text}Field from publicapi.js
	  to src/commands/{math,text}.js (respectively)
Merge branch 'dev' into fix.seek-target
Rename 'edited' hook to more accurate 'reflow'

Also under consideration was 'layout', but that has other meanings
whereas reflow is more specific:
https://github.com/mathquill/mathquill/pull/317#issuecomment-45383191

Just did:

    sed -i '' s/edited/reflow/g $(git grep -l edited)

and rewrote the README entry.

Recommend --word-diff-regex='\w+|\S' to view diff.
Rename API method 'redraw' more specific 'reflow'

The 'edited' hook was originally named 'redraw', and I mistakenly
renamed it in #241 thinking they should mean the same thing, but undid
that in the parent commit.

Just did:

    sed -i '' s/redraw/reflow/g $(git grep -l redraw)

and rewrote the README entry.

Recommend --word-diff-regex='\w+|\S' to view diff.
Merge pull request #325 from mathquill/rename.edited-to-reflow

Rename 'edited' hook to more accurate 'reflow'
Simplify mousemove/docmousemove/selectTo

https://github.com/mathquill/mathquill/commit/deafecd34663057724b45849fc069030f3c25c2c#commitcomment-7541654
Merge pull request #321 from mathquill/feature.innerFields

Feature: .innerFields on static math instances
Merge pull request #322 from mathquill/fix.seek-target

Fix seek target
[LRN] Matrix support

\matrix
\pmatrix
\bmatrix
\Bmatrix
\vmatrix
\Vmatrix
[LRN] Add InertMath: non-editable with no mouse events or cursor

Used to render keyboard buttons so that they don't hijack
focus when they're mousdown'ed.

https://github.com/Learnosity/API/commit/2033856953ff25064bea509371e8b1e304f21e48
[LRN] Add StaticMathWithEditables (for editable areas)

Replicates the behaviour of the old mathquill where the outer
area isn't editable once you add editable zones.

I needed to add an extra class to distinguish inner editable areas,
and the last focused area (so we know where to write when adding
math programmatically).

Arrow keys move the cursor between the editable zones.

https://github.com/Learnosity/API/commit/68d28f93daf85f09035865965f06b2b58abce53d
https://github.com/Learnosity/API/commit/1e098271bcf14331160c01588212a502bef6fa47
[LRN] Check textarea can be selected before trying to do so

Otherwise it'll break when using substituteTextarea to use
a focusable span.