[TASK] Improve signals in ParseNewsletterService

[georgringer]

I checked out the new implementation and I suggest the following changes:

• The new methods parseSubject and parseBodytext have their dedicated signal slot, therefore the introduced BeforeAssignment slot is not needed
• To be able to really use the slot in parseMailText as kind of afterRender, it needs the information if the bodytext or subject is processed.

With those changes it is now possible to:

• use the previous slot for afterRendering and know without guessing if it is bodytext or subject
• have 2 new slots for preRendering and because of its dedicated methods it is also clear which content is processed