Add support for other backends (wtype)

[plattfot]

Add support to add other backends than xdotool. By refactoring the xdotool calls into two function calls.

• do_type: variable containing a function that reads from stdin and type it out.
• do_press_key: variable containing a function that presses the key specified by the first input ($1) to the function.

The xdotool calls now live in _do_type_xdotool and _do_press_key_xdotool which are the default assigned to the do_type and do_press_key respectively.

I also added a backend for wtype as I'm using a wayland wm and cannot use X or ydotool.