Lower CPU usage when fading between departures in the popup icon

- Cache drawn vehicle pixmaps in DeparturePainter.
For monochrome icons, drawing the transport line string (using
QPainterPath::drawPath) into the icon takes the most time.
- Cache drawn polygons for visualizing the time until departure.
- Cache transition pixmaps for fading between departures in one group.
- Lower the animation time for transitions between departures in one
group and higher the pause between these animations.

- Altogether this lowers the approx. CPU usage here (3,4GHz single core)
from ~15% to ~1% while showing the popup icon with fading departures
(tested in plasmoidviewer).