po: update Polish translation
po: update German translation
po: update French translation
Renderer/TrackLineRenderer: Don't check basic.attitude.heading_available

This fixes trac issue #2616.

For the internal "fallback" calculation of the heading heading_available
is never set to true. It is also not possible to set this to true since in
that case the recalculation would be skipped until it expires.

Since we are guaranteed to calculate a heading if basic.track_available
is true that check should be sufficient.
CrossSectionWidget: call ReadBlackboard() after creating XSWindow

Fixes valgrind warnings.
MapWindow: initialise terrain_center with GeoPoint::Invalid()
RasterWeather: initialise center with GeoPoint::Invalid()
TaskManager: RandomPointInTask() returns GeoPoint::Invalid() on error
CrossSectionRenderer: initialise "start" with GeoPoint::Invalid()

Enable the check in Paint().
CrossSectionWidget: move code to method Update()

Updates all of CrossSectionWindow's settings.  Shows up-to-date
information (start and vector) immediately, instead of waiting for the
first blackboard update.
CrossSectionWidget: update data each time the widget is shown

Move Update() call to Show().
Dialogs: add a few "override" and "final" keywords
Screen/ContainerWindow: fix typo