Add showReason method and handle p tags in reason

[reiterl]

Resolve #4345

[reiterl]

I looked at the os-editor. I didn't find the place, where the empty p is added in it. I used the simple solution to check showReason.

[bastianjoel]

While I think your change is needed for backward compatibility I think we should also modify the editor to not output empty paragraphs. (For example to fix the motion form currently accepting empty motions)

[reiterl]

Remove the 'p' tags, if getText() delivers nothing. In cleanOutput.

[Elblinator]

function test