Introduce the ability to navigate using an anchor and aim.
Example use: Set spacecraft as anchor and planet as aim to always look down at a planet, while followin the spacecraft in its orbit.
* Move global objects out of OpenSpaceEngine
* General cleanup of main.cpp
* Add default_joystick asset to all scenes
* No longer suppress mouse interaction on slave nodes
* Window delegate uses function pointers rather than subclassing
* Fix for false overwriting of ImGUI configuration file
* Change default color and tilt angle of fisheye rendering
* Restructured performance manager
* Simplify screenshot handling
* Remove torrent-based synchronization downloads
* Add download URL to error message if HTTPSynchronization download fails
* Add a grace factor to the distance conversion
* Don't use scientific notation when displaying distances
* Introduced guiName to PropertyOwner
* Added requirement that PropertyOwner::identifier may not contain whitespaces
* Changed Name to Identifier in asset and scene files
* Added new PropertyOwner to RenderEngine that owns the ScreenSpaceRenderables
* Moved Name and GuiPath into GUI group
* Added user-facing names to layer groups
