improvement(caret): caret.setToBlock() offset argument improved

[neSpecc]

This PR improves the behavior of the offset argument of caret.setToBlock() method

Right now it uses internal offset of last/first text node. But it should compute offset across the whole block content.

The problem is in use of $.getDeepestNode() — it just returns first or last deepest node. I implemented getNodeByOffset() method what returns deepest text node by passed offset.

Before

// 1234<b>567</b>
setCaret(block, undefined, 6)
// 1234|<b>567<b>

After

// 1234<b>567</b>
setCaret(block, undefined, 6)
// 1234<b>56|7<b>