Add support for Windows

[dgsasha]

• [x] ~~Fix excessive memory consumption. I don't really think there's a memory leak, but the memory usage is extremely high, like 500mb+. At a certain point it starts getting released back to the OS which is a good sign, but GTK4 seems to use way more memory than it should on windows. One possible change that should be made is the windows that are stored in memory after they are closed maybe should be destroyed instead of hidden. Not sure if this would be better or worse.~~
• [x] ~~Use the windows file picker. The GTK file picker is a little buggy on windows and looks out of place so maybe switching to a windows one would be better.~~
• [x] ~~Fix notification actions. I can't get the notification actions to activate the app through the command line, so right now the notifications do nothing when clicked. Unsending notifications also doesn't work.~~
• [x] ~~Add CI for windows builds.~~