Separate searches for to, cc and bcc fields and uids

Added separate searches for to, cc and bcc fields which can for example be useful if you want to filter whether someone is only addressed on the cc line. 
Furthermore, with the ability of searching by uids there is now the possibility to make a more efficient filter if you want to filter only on new messages since the last search by storing the highest uid and use this as starting point for the next search.