Refactor and other miscellaneous changes

[tfuxu]

Description

This PR contains a Paleta codebase refactor and some other miscellaneous changes.

Included changes:

• Refactor Rust modules
• Move Flatpak manifest to build-aux and change a path to point to the root directory
• Add .editorconfig and format some top-level files with it
• Add DOAP metadata (required in GNOME Circle)
• Use action GLib macro instead of gio::ActionEntry::builder function
• Add Ctrl+W shortcut for closing Paleta's window
• Remove color-thief-py from acknowledgements section in About window
• Stop using i18n_f function in some remaining strings and make delete_palette_dialog heading string translatable
• Move most configuration stuff to the main meson.build
• Merge run and setup shell scripts into new local-build.sh
• Update gdk-pixbuf crate to 0.16