Signature at message start

- This modification puts the signature (i.e. sender name) at the message start.
- Any separators have to be contained in the signature.
- While the sender name field in signature configuration has focus, a tooltip with an example result of prepending the signature to the message is displayed.
- After the first character has been typed into the sender name field, a colon is added to the right of the caret.
- Message size indicators and message part coloring are modified accordingly.
- The signature is prepended only if it is not already present at the message text start. Only whitespace is stripped for the purpose of comparison to allow signatures composed entirely of non-alphanumeric characters.
- Automatic upgrade appends a colon to all non-empty signatures.
- One localization string for translation has been added.