osgearth script improvements

[hamish]

Hi,

while testing the packages I've come up with a few comments/improvements which would be nice for the Osgearth installer script:

• mention in the quickstart that it might not like running in VMs (OpenGL req'd)
• and that a network connection is needed for WMS and OSM overlays to work

• add a desktop link to the simple Blue Marble example, then it can be in the menu and get a bit more exposure.

thanks, Hamish