Vertical movement of lifelines not always possible and causes problems during layouting

[Matthias Schoettle]

Given a large message view, there seem to be two problems that might or might not be related:

• metaclasses can be dragged down, but will jump back up to their first position
• panning the message view causes lifelines that were moved down to move somewhere else on relayouting the message view

[Matthias Schoettle]

References #513: Adjusts the first spacer directly instead of triggering a complete layout of the message view.

→ <<cset 8be654f650b9>>

[Matthias Schoettle]

References #513: Starts with the gloal y-position of the first lifeline (target) when layouting.

This prevents the message view to "jump back" when re-layouting after the message view was panned.

→ <<cset 2dd0df234d50>>

[Matthias Schoettle]

Resolves #513: Triggers layout at the end of the drag gesture.

This allows any message calls that are drawn improperly to be corrected.

→ <<cset 199a66f8e7b7>>