Improve Message View user experience

[Matthias Schoettle]

The following points should be done to improve the user experience of message views:

• It is hard for the user to see, where new fragments or messages are allowed to be created. These places (the spacers on the lifelines) could be highlighted.
• When editing a message view, there is a limited choice of buttons in the menu, the save button should be part of it.

[Matthias Schoettle]

References issue #234: Added the visualization/highlighting of spacers on lifelines where creation of messages or other fragments are possible.

→ <<cset 71ed95448b2d>>

[Matthias Schoettle]

References issue #234: Added save button to message views, but in general to all views that display a back menu.

→ <<cset 18f877f125db>>