Masterbar: integrate into existing wp-admin toolbar instead of overwriting it

The current Masterbar implementation overwrites the whole wp-admin toolbar. That can be problematic since other plugins often add elements to that default admin bar, and those elements disappear when the Masterbar is activated.

Ideally we would follow core standards and add our elements to the default toolbar like others do, to play nice with other plugin authors.

Internal discussion about Masterbar improvements: p9Jlb4-R8-p2

Start by locating the current Masterbar implementation and how it replaces the wp-admin toolbar; the issue names no file or test. Review the internal discussion p9Jlb4-R8-p2, then verify that existing plugin toolbar elements remain while the Masterbar elements are added to the default toolbar.

Written by the indexing model from the issue text.