Clean up scripts, generate bindings from same Gir files every time

[stackotter]

This PR cleans up all of SwiftCrossUI's existing bash scripts. It;

• moves them to the new Scripts directory,
• pulls them apart into smaller self contained scripts (e.g. generate_gtk3.sh),
• adds extra code that ensures the scripts can be called from anywhere,
• optimises Gtk binding generation by running the format script directly on the generated code (avoiding formatting the whole repo),
• and generates the Gtk bindings from a consistent set of Gir files (hosted in https://github.com/moreSwift/gir-files)

You can now regenerate all generated code by running ./Scripts/generate.sh; no need to run ./Scripts/format_and_lint.sh after the generation script anymore. You can also regenerate only the code you changed by using ./Scripts/generate_gyb.sh, ./Scripts/generate_gtk.sh, ./Scripts/generate_gtk4.sh, and ./Scripts/generate_gtk3.sh.

[stackotter]

Addresses #259