Some more debug output regarding textures

Should help to analyze situations from log files, especially remaining
textures.
Config changes for images + minor logging stuff
A bunch of smaller changes...

- added a config option "DisplayDebugUI" to hide/show debug UI
- added a config option "DontLoadEditorCatalogThumbnailParts" to prevent
loading parts not actually used on a rocket while in editor
- added toolbar support for the DebugUI (if enabled via config)
- Implemented a first version of an external API
- Preperation for a new IO buffer and upscale request queue
Most work towards v3

- external api improved
- load requests can now be canceled (as long as still queued)