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
Merge branch 'dev' into fix.seek-target
Simplify mousemove/docmousemove/selectTo

https://github.com/mathquill/mathquill/commit/deafecd34663057724b45849fc069030f3c25c2c#commitcomment-7541654
Merge pull request #322 from mathquill/fix.seek-target

Fix seek target