Minimal implementation of missing setHoverEnabled

The AnnotationEventCallback::setHoverEnabled(bool) method is not
enabled. Implement a minimal version to allow for setting the
_hoverEnabled attribute.

Does not disable hovering if there are items that are currently hovered.
Incorrect altitude mode in TerrainProfileGraph

The GeoPoint start/end attributes in the TerrainProfileCalculator are
relative, but the TerrainProfileGraph tries to convert the start/end
points toWorld() coordinates, which requires absolute altitudes.

Added overloaded methods to the TerrainProfileCalculator to request the
start/end points in a specific altitude mode, and changed the
TerrainProfileCalculator to use the new TerrainProfileCalculator
interface.

Resolves Issue #532 in the osgearth master.
Increase Tessellation of Terrain Profile Line

Increased the tessellation of the line on the terrain to more precisely
follow the terrain profile when using higher resolution elevation
models.
Merge pull request #534 from michaelarice/master

Updated TerrainProfileGraph and AnnotationEvents