fix problem with clearing non-empty location field after changing type of config variable

introduced by cd9db17 ("clear variable location cell in variable
panel, if variable class or type changed").

Closes #26

.project

.directory

syntax: regexp
^tests/.*/build$
^.idea/.*
syntax: regexp
^.*\.pyc$
syntax: regexp
^.*~$
syntax: regexp
^.*\.swp$

bug_report.*\.txt
i18n/.*.new$
revision