Separate input handling from rendering

To minimize rendering, the input must be handled in a separate thread.
To see, why, consider the optimal rendering solution: renders are only
necessary when the pty has data that changes the terminal state, OR
there is a window event which changes the graphics state. When not
drawing, the render thread is to remain parked at a condition variable,
and it's not possible to handle input while parked! Thus, we need a
separate thread.

In addition to adding the separate thread, each subsystem thread is now
spawned in a separate function to (hopefully) improve readability.

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