It prevents unwanted implicit conversions from e.g. nullptr to Camera,
Vector2 to Physics::Point etc. By making all the constructors explicit
it is easier to routinely add the keyword to all new classes instead of
thinking about cases when to add and when not to.
Because with NaCl WindowContext doesn't make sense anymore, the classes
are now renamed:
* Contexts namespace -> Platform
* *WindowContext -> Application
Hopefully this is (one of) last crazy renaming runs.