Improve look of the popup icon,

- Update DeparturePainter with new flags, updated functions. The new
flags can be used to toggle the use of monochrome icons, toggle
drawing of the transport line string in an "empty" vehicle icon,
toggle visualizations of the time until the departure of the
currently shown departure.
- New class PopupIcon that stores data for the popup icon of the applet
and handles animations between departure groups and between
departures in the current group. Some code came from
publictransport.h/cpp.
- Update vehicles.svg.
- The popup icon now only shows one departure at a time. If there are
multiple departures in the current group, the popup icon fades
between these departures.
- The popup icon also visualizes departure times differently: It fades
departures radially in (departure in 0 minutes => completely opaque,
in 60 minutes => completely half-transparent, in 15 minutes => the
top left part becomes half-transparent, etc). Can be intuitively read
like a clock and looks similiar to the visualization of running tasks
in the notification plasmoid, making PublicTransport look very much
at home in the systemtray plasmoid.
- Important information about the next departures can now be read
easily from the popup icon (approximate time until departure, vehicle
types and transport lines).