Improved unicode conversion

Changes:

* Newly typed sequences are recognized anywhere, not only at the end (useful when revising your message before posting)
* All transformations are ignored inside URLs - fixes #167
* Undo function works even if text was shifted in the meantime
* Undo steps are shown only in the message area they belong to, not leaked into other replies
* Undo button can now show every symbol (fixed button font bug)
* Fixed undo button style in theme_calm