Add Smiley Buttons in Signature Editor

When editing the signature, the smilies may be allowed. But the smiley buttons are not displayed in the signature editor so that the user has to know the smiley code to enter it manually. This is not as user-friendly as the remaining of phpBB.

This improvement adds the smiley buttons in the signature editor when smilies are enable. It also add the links to the extra smiley pop-up window, if any.

Note that this improvement has an impact on the templates. Therefore unmodified templates of a custom style are likely to behave according to one of these possibilities:

• No Change: The smilies are not added but all works fine as before (unmodified Subsilver behaviour).
• Partially functional: The smilies show up and work. But if there are extra smilies, the pop-up window can be open but it won't be able to add the extra smiley in the editor (unmodified Prosilver behaviour).

In attachment the SVN patch made under Windows (with CRLF, sorry)