Handle alpha and space chars as raw events

To handle special chars, text is handled as text input instead of key
events. However, this breaks the separation of DOWN and UP key events.

As a compromise, send letters and space as key events, to preserve
original DOWN/UP events, but send other text input events as text, to be
able to send "special" characters.

Fixes <https://github.com/Genymobile/scrcpy/issues/87>.

Suggested-by: pete1414
Suggested-by: King-Slide <kingslide@gmail.com>