Merge IMP_Mailbox_List and IMP_Mailbox_List_Track
Remove IMP_Mailbox_List#getIMAPIndex()

It is duplicative of the array access to the object.
Also, IMP_Mailbox_List#getMessageIndex() -> IMP_Mailbox_List#getIndex()
Move search related code from IMP_Mailbox_List to extended IMP_Mailbox_List_Virtual class