input: use events instead of threads

1. Removes limitations on a number of input devices

#define MAX_KEYBOARD_DEVS 4
#define MAX_POINTER_DEVS 4
#define MAX_TOUCHSCREEN_DEVS 1

2. Fixes half-connected input devices

     * FIXME: Some libinput devices (notably, certain hid-i2c keyboards)
     * report both keyboard and pointer capability. Since lvgl expects
     * every input device to have only one type, we register those devices
     * as keyboards, considering this capability more useful. However
     * we must also assume that keyboard+touch devices are touch.

3. Adds support for multiple hardware keyboards (a built-in and an external keyboard, for example)

4. Adds a configuration section for hardware keyboards

5. Adds support for hardware power buttons

Fixes #52 (closed)
Fixes #18 (closed)
Fixes #51
Fixes #7 (closed)
Fixes #16 (closed)
Fixes #23 (closed)