Enhance elementary OS app template

[dar5hak]

The eos template is great for a barebones app, but there are a few more assumptions we can make:

• Granite dependency in meson.build (Add dark style support for elementary OS template #12)
• Dark style support using Granite settings + AccountsService (Add dark style support for elementary OS template #12)
• A meson/post_install.py for installing resources
• A GitHub Actions workflow for linting and building the Flatpak (Add GitHub Action to lint and build Flatpak for eos template #14)
• Translation setup
• .gitgnore
• .editorconfig (create template with .editorconfig #24)

See https://github.com/elfenware/obliviate/ for a reference implementation of all of these.

I will get around to working on this in a few days/weeks, but leaving the issue open for anyone else to pick up in parts or whole.

[dar5hak]

Note 1: Some of this might even make sense for the gtk template

Note 2: Is it too late to rename eos to elementary-os? The former is unofficial, and maybe unclear to some. (https://elementary.io/brand) I saw the discussion about this in #7. Cool then.