[R-Model] Fixed missing implementation in FilteredFrame

Signed-off-by: Stephan Wahlbrink <sw@wahlbrink.eu>
[*] Reverted changes of 0a7f52fa8b6073b6c72f6c9f68ef202b8c1d4b28:
    Adapted to changes in ECommons-UI (StringVariableUtil)

Signed-off-by: Stephan Wahlbrink <sw@wahlbrink.eu>
[LTK-Model] Added #toString to generic source unit implementations

Signed-off-by: Stephan Wahlbrink <sw@wahlbrink.eu>
[R-Moble, R-Refactoring] Fixed "rename in workspace" refactoring (element
was renamed twice).

(reported by Vlad Sharanhovich)

Signed-off-by: Stephan Wahlbrink <sw@wahlbrink.eu>