Rewired "Available" vs "Enabled" MultiTargets and TargetFrameworks

[Arlodotexe]

This restructures our props to creating a clean distinction between these four traits:

• Available or Enabled
• MultiTarget or TargetFramework

1. Available TargetFrameworks. A list of static values declared by the tooling, these are the TargetFrameworks that can be enabled.
2. Available MultiTargets. A list of static values provided by the component or current project. These are the MultiTargets that the component can support.
3. Enabled MultiTargets. Dynamically provided by the user each time the solution or build scripts run.
4. Enabled TargetFrameworks. Dynamically determined using 1, 2 and 3, these are the TargetFrameworks that get enabled for build.

This "lifting" of the TFM toggling above the TFM declaration and into the MultiTarget system enables overriding "Available" TargetFrameworks per-project, while still allowing it to be disabled if not supported (2) or requested (3).

[Sergio0694]

Looks much cleaner 🙂

[Arlodotexe]

Needs #234