Don't allow to open more than one filter editor for a single filter simultaneously

[Mikhail Lopatkin]

See the issue #121 for some background.

1. Create a filter
2. Open an edit-filter dialog for this filter
3. Open an edit-filter dialog for the same filter once more

Expected result: no new dialogs are opened, first dialog is activated

Actual result: new dialog opens. The first dialog to close is the one that updates the filter, the result of the second one is ignored.