Implementing the Database screen, bug fixes.
- DatabaseScreen class added, database.html and database.css filled with content
- resource management improved: shaders and cubemaps only get loaded upon request
- GL management improved: vertex buffers only get reinitialized if new geometry was added (ManagedGLContext is now Resource)
- possibility to add a spacecraft to the scene using only a specific LOD
- contexts are now created with alpha parameter to allow transparent background of canvases
- camera matrix handling in scene.js improved
- spacecraft types added
- information about the spacecraft classes added to the XML
- Egom models now calculate and save all three dimensions (of their size) upon loading
- several smaller changes and one-liner bug fixes