New modding patch : BaseFieldListUseFieldHost

[gotmachine]

This was extracted and refactored from the larger #118 PR.

The purpose of this patch if to allow BaseField and associated features (PAW controls, persistence, etc) to work when a custom BaseField is added to a BaseFieldList (ie, a Part or PartModule) with a host instance other than the BaseFieldList owner. This allow to dynamically add fields defined in another class to a Part or PartModule and to benefit from all the associated KSP sugar :

• PAW UI controls
• Value and symmetry events
• Automatic persistence on the Part or PartModule hosting the BaseFieldList

Potential use cases for this are either :

• Having a part or module-level PAW item associated to and sharing the state of a common field, for example a field in a KSPAddon.
• Extending external (typically stock) modules with additional PAW UI controls and/or persisted fields.

Sample code demonstrating those use cases is provided in the source file, after the patch.