issue-#161 fixing Dialog.MapString with empty message

[Josef Templ]

I have created issue-#161 for the problem reported by Helmut in
viewtopic.php?f=40&t=624.

The fix includes Helmut' suggestion.
There was also a bug with empty subsystem names e.g. "#:xxx"
reported a missing resource file. This has also been fixed.
Also the docu is slightly improved by pointing out that there is only a mapping
when Subsystem and message are non-empty.

For the diffs see https://redmine.blackboxframework.org/p ... c&type=sbs.

- Josef

[Zinn]

Looks good. Everything works well.
It is ready for voting.
- Helmut

[Josef Templ]

I agree.

- Josef