Added ClearPartBindings to BindingSyntax

[steinbitglis]

Description

This adds a method to BindingSyntax that removes/clears all the part bindings of a composite binding.

Changes made

The added method (ClearBindingParts) replaces code similar to this:

        void ClearBindingParts (BindingSyntax b) {
            if (b.valid)
                b = b.NextBinding();
            while (b.valid && b.binding.isPartOfComposite)
                b.Erase();
        }

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

[steinbitglis]

I guess that in theory it could be an extension method, but it uses a lot of internal fields and stuff, so it makes more sense a proper method.

[jimon]

Hi, thanks for your contribution! We gonna proceed with merging it with slight tweaks in #1641