Support loading extra libraries during build

[andydotxyz]

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

Some developers want to load extra libraries when building their app (like #102), we don't want to hard code them in so this could be extensible somehow instead.

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

No, at least not without custom images.

Describe the solution you'd like to see:

A metadata file or addition to FyneApp.toml that can list libraries in a way that can be interpreted for the various OS where it would apply.