Bug #689806 - [webkit-composer] Save as draft forgets signature position

When opening message that has signature with 'Edit as new message', no signature
is added because in signature combo box is active signature set to None. We need
to set active signature id in combo box to that signature that message contains.

Also insert signatures through DOM manipulation, because we don't want to insert
empty lines that WebKit inserts through 'insertParagraph' command.
Bug #689782 - [webkit-composer] Cannot start new numbered list below other one

When inserting new ordered list under existing one it was continuing in previous
one. This behavior was caused by changing the format of list but again to ordered
list (numbered, alphabetical, roman numeral lists are treated as one in WebKit).
So we have to insert these lists manually in composer through DOM modifications.
Allow wrapping for paragraphs with formatted text (bold, italic, ..)

Also most of the code was rewritten to simplify the wrapping. Also when
paragraph formatting is changed to 'Normal' we are immediately marking it
for wrapping.