Multi-monitor support

[s0]

closes #10

There are a couple of commits I also added to ease development a little bit (638c8524ebc6cc429968dd14faebf31034a410a8, 0d16e9dfa256c7b1dcc6d0fe42b5d685b666494d), I can more them to another PR or just remove if you'd prefer.

This is an initial implementation for multi-monitor support, that seems to be working quite well on my setup. I've included the ability for themes to get metadata about their window, including whether or not they're primary, and have hidden the login components for non-primary screens on the default themes.

I haven't yet implemented cross-window communication, but wanted to get this PR open as a draft before that to make sure that you're happy i'm on the right track. That's the last remaining part, and i'll use the dracula background-switcher as a demo of this functionality.

TODO:

• [x] Implement cross-window communication
• [x] Decide on mechanism for themes to pick per-window URLs
• [x] Fix sending metadata on theme refreshes

[s0]

Thanks for the initial review! Also noticed you introduced linting and formatting, very nice to see! I've incorporated your suggestions in a rebase, and also ensured that it's formatted + linted throughout the history.

I also spotted a bug where if a frame is reloaded (via ctrl+R, or via theme reloading etc...), then it won't initialize as metadata isn't sent more than once, so i'm going to look at fixing that now by having the theme frame request the metadata and sending it then, tracked this in a TODO above.

[s0]

Okay I've now updated the PR addressing the refresh bug, and allowing cross-window communication via a broadcast mechanism. This mechanism is then also used in the dracula theme to apply wallpaper changes across all screens.

Other than the open question around handling of a secondary URL, i think this is ready to go now :)

[JezerM]

Now, nody-greeter will look for index.theme (still not sure about the name) inside the theme's directory, if it does exist then it will be loaded and used to set primary_url and secondary_url for all monitors. However, if the secondary_html property does not exist, it will fallback to the primary_html.

[JezerM]

I just added the secondary.html and index.theme for both gruvbox and dracula themes.

Added a "fake" display to test it locally, and yeah, it works!

https://user-images.githubusercontent.com/59768785/149647671-5e6356b1-8284-4c5f-bd87-baa8b5109d4d.mov

[JezerM]

io-ts looks interesting, powerful and easy to use. I did a little implementation with both greeter and theme's config, but I'd like you to check if this is correct or there's any improvement to be made.

[s0]

Just my one comment regarding undefined above probably needs to be addressed, then i think this is ready to go! :) Happy to defer the conversation around sync / async I/O operations to #13

[JezerM]

Nice~ It looks better and simpler. Now, I'm going to check if there's any issue or anything to be improved before I merge this PR.

[s0]

Nice~ It looks better and simpler. Now, I'm going to check if there's any issue or anything to be improved before I merge this PR.

Sweet! Excited to get this in! And thanks for being an easy maintainer to work with!