Incorporate build settings from each target in a scheme

Resolves #128 and #91 (incidentally).

Note that we still want to use _schemes_ as the primary level of abstraction, because they come with additional configurability on top of targets, and we want to match what you'd do in Xcode as closely in possible. The important difference is that we need to use the built products for _each framework target_ in a scheme.

/cc @RuiAAPeres

**To do:**

 - [x] Generate one `BuildSettings` value per target in the chosen scheme
 - [x] Copy build products from each framework target built
 - [x] Test adding a dummy framework target to a RCL scheme
 - [x] Test adding unit tests to a RCL scheme
 - [x] Specifically ignore build products from non-framework targets