Run wal on background change and use system sassc

[daniromome]

Had a few issues getting the tweak to work with the sass installation button, I made it so it uses system-wide sassc and also added automatic run of pywal whenever background changes since pywal color schemes are supported by a handful of applications. Attaching screenshot of what kind of themes could these changes enable

[ardjuh]

Please update the readme and the install sass button in prefs (to install sassc and pywal or point to the githubs to install them)

[ardjuh]

At a future moment, we can maybe generate a pywal theme and apply it, so it's more coherent with the rest of the theme and has better readability.

[aidanlw505]

The way this PR checks if wal is installed will cause issues on certain systems because it only checks /usr/bin/wal. It assumes it's installed globally, when some users might install it in their user path. Even when I install it via pip3 with sudo it ends up in /usr/local/bin/wal on Fedora. I would suggest to check in $PATH

I would also suggest adding a check to see if wal should be ran with -l to potentially generate better color pallets for light mode.