iOS 0 length range detection revamp

[szydlovsky]

I found a few really strange issues that resulted from previous assumptions; previously I thought detecting a style at a 0 length range was equivalent to cursor being there. Later code proved me wrong (mainly zero width space logic) and I decided to revamp the way it works.
Now:

• if cursor is at the place, just use typing attributes to retrieve needed values
• otherwise;
  • if the place is as the very end of the input, either there's no style (inline styles) or check the first character of the paragraph if it exists (paragraph styles)
  • otherwise check attributes at the given index using apple's API for this

Apart from that, this PR also fixes a situation where backspacing a list or blockquote wouldn't remove the style.