Added initial cut of the osgearth_package_qt tool.
Added support to the osgearth_package_qt tool for loading a blank globe and removed the default earth file.
Removed sky node and fixed logging bug.
Added bounds specification and a progress bar for export.
Added multithreading (per-layer) for improved packaging times.
Added support for multi-select of files.
Added missing image files.
Removed extension override option in the osgearth_package_qt tool.
Merge branch 'master' into topic-tilertool