This modifies `input_cmd_scroll_button` to utilize the mouse button
helper `get_mouse_button` when parsing the button. x11 axis buttons are
not supported with this command and `CMD_INVALID` will be returned, but
all other x11 buttons, button event names, and button event codes should
be working
Now the scroll_button will not accept:
- letters on string beginning;
- negative numbers.
What is tolerated:
- letters after number;
- rational numbers: the fraction after dot will be omitted.
This commit introduces a scroll_button option, which is intended to be
used with scroll_method. Now user can edit his sway config and add an
scroll_button option to device section.
