main.c: add a real-time signal for toggling visibility of the keyboard

Previously it was only possible to either show or hide the keyboard.
When you want to control the keyboard with a single button in, i.e.
a window manager, this requires a separate script to keep track of
state. This solves this by only needing one signal to toggle.

Since there are only 2 USR signals, I had to use real-time signals,
which are in the [POSIX
standard](https://standards.ieee.org/standard/1003_1b-1993.html) and
available in the [Linux kernel since
2.2](https://www.man7.org/linux/man-pages/man7/signal.7.html).

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