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.
Brian Ashworth
Robert Kubosz