Allow to move lifelines vertically

[Matthias Schoettle]

Sometimes, when lifelines start later within a message view, it is preferable to move the lifeline down in order to maximize the available space horizontally.

Therefore, it should be possible to move them vertically, but not lower than where the first message starts.

[Matthias Schoettle]

Resolves #498: Adds support for vertical movement of lifelines.

Allows y movement within a range of the default lifeline position and until when the first message on the lifeline begins. Fixes determination of sizes by using global ones.

→ <<cset 4aeec6ff38f4>>