Integrate fyne-cross and fyne CLI

[Bluebugs]

Is your feature request related to a problem? Please describe:

fyne-cross need to follow fyne behavior closely and having two different repository increase complexity in managing two different release cycle. Integrating fyne-cross into fyne, could also lead to better developers experience by probing the system and falling back to fyne-cross when cross compiling. It could also improve functionality discoverability by having only one command and tool to install.

Is it possible to construct a solution with the existing API?

It is, by calling fyne-cross and fyne independently, but it isn't giving the best possible experience.

Describe the solution you'd like to see:

This issues is about figuring out a path forward to merge fyne-cross into fyne. Figuring out what we want to improve in fyne-cross code base prior to merging and then what are the step to do that merging.

Rough idea here:

1. Migrate fyne-cross to use cli.App
2. Move Docker image to an independent repository (would be nice to automate their build on release)
3. Address the darwin image specificity
4. Change fyne-cross path to match fyne CLI path
5. Import fyne-cross git into fyne CLI
6. Plug fyne cross command in
7. Add logic to automatically detect if the system should fallback to fyne cross
8. Release fyne 2.3
9. Archive fyne-cross git repository
10. Enjoy :-)

Let me know what you think and if that path make sense to you.