Add RFC: Separate context bar code into UI and plugins

[norihiro]

Description

This RFC proposes to separate the implementation of the context bar into UI/ and plugins/.

Motivation and Context

UI has the implementation of the context-bar for each source type.
As the result, property names are hard-coded in both UI and plugins.
In possible future modification, it might require to change both plugin and UI, which might lead a potential bug.

It is a good implementation to separate the plugin-specific implementation from the UI.

This will also enable 3rd party plugins to have their context bar.

Rendered RFC