Changes the interface as well to remove any unique methods that
frontends needed to call such as StartJoystickEventHandler by
conditionally starting the polling thread only if the frontend hasn't
started it already. Additionally, moves all global state into a single
SDLState class in order to guarantee that the destructors are called in
the proper order
Modules didn't correctly define their dependencies before, which relied
on the frontends implicitly including every module for linking to
succeed.
Also changed every target_link_libraries call to specify visibility of
dependencies to avoid leaking definitions to dependents when not
necessary.
James Rowe