1. Add `:event_handler` app option. It is a process that will receive
the app events, the `:open_app`, `{:open_url, url}`, and
`{:open_file, path}` messages.
2. Add `AppBuilder.init/0`. This reads the `APP_BUILDER_INPUT` env
variable which contains the app event the release was started with.
3. Use <https://github.com/wojtekmach/otp/tree/wm-WX_MACOS_NON_GUI_APP>
branch which contains wx fix that hasn't been merged yet.
1. Replace `mix release mac_app|mac_app_dmg|windows_installer` with a single `mix release app`
2. Extract templates (Launcher.swift, Launcher.vbs, etc) into separate
files in app_builder/lib/templates
3. Don't verify vc_redist.x64.exe checksum as the Microsoft publishes
new releases on the same URL
