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.
- Restore old one if we weren't part of a block (should be NULL anyway)
- Check current_input_config got properly allocated
- free temporary current_input_config when done using it
Some users may want to switch buttons on their input devices, turns out
libinput already supports it. Let's add a support for it in our config.
Signed-off-by: Michał Winiarski <knr@hardline.pl>