correct handling of lefthanded coordinate orientations in mitkPlaneGeometry.

Amend ensurePerpendicularNormal() and InitializeStandardPlane() to check for
lefthanded coordinate systems and set the normal vectors sign accordingly,
since those methods ignored lefthanded geometries before.
Extending CppUnitTests mitkPlaneGeometryTest.cpp with TestLefthandedCoordinateSystem().
Clarifying comments and cleaning up coding style a bit.
This patch fixes #11477, #16772 and #18114.