Given the way GetPromotedPoint / IsNodeInActiveEditor are designed, when the
selection is in the first child element of the active editing host the
selection is extended outside of editor node.
As a result, `GetNodesFromSelection' (which is called at the beginning of
`WillAlign') returns two nodes:
1. an empty text node, which is enclosed in a <div style="text-align: ***">;
2. the first child element, which is destroyed when it's a <div>;
Proposed solution:
#1 can be avoided by checking that the node is editable in `WillAlign';
#2 can be avoided by not dropping <div> blocks in `RemoveAlignment'.
This is an opportunity to simplify `RemoveAlignment' a bit.
