Docs UI support improvements (#1916)

* FIX: Eliminated nullref exceptions via checked access of referenced actions since they may not exist. Added editor and run-time warnings to detect non-compliant UI actions.

* Improved test coverage and fixed issues in verifier.

* Added CHANGELOG.md entry

* Removed obsolete test code

* Adding missing symbol check for Unity 2022.3

* Formatting fixes

* Formatting fix

* Corrected verifier constraint on unconstrained expected control types which based on rebinding code seems to be implicit Button type.

* Eliminated need for conditional editor compilation symbol

* Added missing editor guard

* Removed passing internal field by ref with direct access.

* Added reporting policy support to either report all or suppress child errors. Also updated test accordingly. This allows just changing the setting (constant at the moment) to change reporting mode.

* docs improvements to UISupport, WIP

* continued improvements WIP

* Continued improvements to UI page

* whitespace formatting fix

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Co-authored-by: Håkan Sidenvall <hakan.sidenvall@unity3d.com>
Co-authored-by: João Freire <joao.freire@unity3d.com>
Co-authored-by: Paulius Dervinis <54306142+Pauliusd01@users.noreply.github.com>

Author: 4 people