Fixes #13, fixes #14, fixes #15

* Catch exceptions from password manager modifyLogin() and display as popups,
  as exceptions are from other similar calls.
* Improvements to editor data validation and error prevention:
** Validate Host field as well as Submit prefix or Annotation (depending on
   login type) to make sure they are not empty when about to add or modify it;
   if one of these is empty, display an appropriate popup error, and refuse to
   proceed.
** When multi-editing, disable the Submit prefix and/or Annotation fields,
   depending on what types of logins are being edited, in order to prevent
   duplicates that would arise by setting those fields.

Needs translations in pwdedit.properties