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
JS: add geometry.buffer() function
Merge branch 'master' of github.com:gwaldron/osgearth
Fix: precision loss in ImageUtils GLTypeTraits scale factors
duktape: separate geom bindings; dfix buffer fun; add getBounds func; add examples.
Fix: error when looking straight down in PixelAutoTransform
Handle invalid start/end on recompute

Clear the terrain profile when recomputing using invalid start/end
points.
TerrainProfileWidget enhancements to clear/copy data

Added a “clear profiles” button to the widget, which clears the undo
stack, clears the graph, and removes the line/marker from the map.

Added a “copy to clipboard” button which copies the current graph
image, including the hover line, to the clipboard. Also copies the
elevation profile data to the clipboard in CSV format, such that it can
easily be used in other applications, such as a spreadsheet. The
start/end point coordinates are copied to the clipboard and are
formatted using the formatter set on the graph.
Skin: add skin-name; fix geometry js issue
Merge pull request #535 from michaelarice/master

TerrainProfileWidget/Graph Enhancements