Simplify and improve documentation of logic for forwarding messages from app UI to Firefox extension core

Much of what panel_content.ts does is forwarding RPC messages
from the addon's UI to the priviledged extension code which then
uses priviledged APIs to perform an operation or forwards the request
on to the active tab.

Remove some duplication for these calls and add documentaton to clarify
where each RPC handler receives its requests from or sends them to.