Add a popup MarbleWidget to StopLineEdit

- The popup functionality is implemented in PopupHandler
- PublicTransportLayer is used to show positions of suggested stops
in a MarbleWidget, it also shows some selected stop names
in annotations and allows to hover/select stops
- PublicTransportMapWidget derives from MarbleWidget and uses
PublicTransportLayer, provides additional functions/signals,
implements the hovering
- The use of Marble can be disabled completely at compile time using
the BUILD_MARBLE_MAP cmake option
- Add coordinates to the Stop class and a boolean
hasValidCoordinates, the map can only show stops when this is true
- Add comparison operator to Stop for automatic sorting of stops
to a good order for drawing (in the QMap)

TODO: There is currently no provider in GIT that supports this,
I'll commit one soon.