Fix selecting messages in mailbox view
Get rid of IMP_Ui_Folders

Move import/generate code out of IMP_Ui_Folders/IMP_Mailbox and into its
own separate IMP_Mbox_* classes.
IMP_Ui_Imageview -> IMP_Images
IMP_Ui_Editor -> IMP_Editor
IMP_Ui_Headers -> IMP_Mime_Headers
IMP_Ui_Contacts -> IMP_Contacts
Remove unneeded mode parameter to two hooks

Not only was it using the obsolete 'imp' and 'dimp' identifiers, it is
not needed since this info can be obtained via Horde_Registry#getView()