Make it easier to convert an existing phoenix app into a desktop app

[dominicletz]

This is feedback I've received from a user. Right now the steps to convert an existing phoenix app are undocumented and potentially difficult.

We should probably document the process and also reduce friction. Some initial thoughts:

• The explicit usage of Desktop.Endpoint might not be needed (anymore) with more recent phoenix apps
• The explicit Desktop.Window creation could benefit from a simpler default.

A write-up of the minimal steps is probably a good first step here.

[adamwight]

In #62 we added an Igniter script which can do a large chunk of the necessary changes, but it still needs some improvements, and once we feel comfortable with its operation we should add user documentation.

After the next hex release it will become possible to run the igniter script in an existing project, like so:

mix igniter.install desktop

+1 that documenting the steps is key. I did find a few gotchas when converting a medium-complexity existing Phoenix app, and I'm realizing that in addition to doing conversions, the installer should also detect edge cases which might make the project incompatible without further changes. For example, runtime.exs is unsafe at the moment since it usually depends on environment variables which won't be available in some environments such as in the mobile apps.