WindSettingsPanel.cpp: fix segv in Advanced Config Wind

When edit_trail_drift was false, SetVisibility() was crashing
NEWS.txt, version bump 0.1.5
README: update for Top Hat
GotoTask.cpp: set pointer to nullptr

fixes bug where test for !tp was returning incorrect results
crash after takeoff, landing, takeoff again.
GlidePolar.cpp: reduce takeoff speed to 1/3 VMIN

This works better for slow flying paragliders especially if
no wind est exists.