* net/xesam.el (xesam-all-fields): Remove source and content

* net/xesam.el (xesam-all-fields): Remove source and content
identifiers.
(xesam-dbus-unique-names): New defvar.
(xesam-dbus-call-method): New defun.  Replace all calls of
`dbus-call-method' by `xesam-dbus-call-method'.
(xesam-get-cached-property, xesam-set-cached-property): New
defuns.
(xesam-get-property, xesam-set-property, xesam-refresh-entry)
(xesam-new-search, xesam-search): Apply
`xesam-get-cached-property'.
(xesam-search-engines): Make it an association list.  Take changed
layout into account in the corresponding function.
(xesam-delete-search-engine): Remove check for consistency of
`xesam-search-engines', not needed anymore.
(xesam-mode): Show XML query string only in the debug case.