Add Pywal theming

[aidanlw505]

This is a more complete implementation of #56. It supports light mode by running wal with -l when you don't have dark mode enabled. It also includes an installation button for Pywal via pip3. I couldn't find a good way to check if wal is installed, so as is, the install button is persistent regardless if Pywal is installed. If anyone has suggestions on how to better handle this, just let me know. There is also a toggle to turn on and off Pywal theming. Lastly, I added a section in the README to help users if they've never used Pywal before.

[ardjuh]

Looks quite nice!
On https://lazka.github.io/pgi-docs/Gio-2.0/classes/Subprocess.html you can find a lot of functions to check if pywal is installed.
Something like this should work:

const process = Gio.Subprocess.new(
    ["wal", "--version"],
    Gio.SubprocessFlags.NONE
);
process.wait(null);
return process.get_successful();

[FrancescoCaracciolo]

Fixed conflicts, and checking if pywal is installed, I'm finally merging this