Main thing: Making contextMenus implementation webext-oop compatible.
Preparation:
- Add getParentEvent to ChildAPIManager to allow use of remote events.
- Introduce `addon_parent_only` to "allowedContexts" to only generate a
schema API in the main process.
- Do not fill in `null` for missing keys if the schema declares a key as
`"optional": "omit-key-if-missing"`. This is needed for the second
point in the next list.
Drive-by fixes:
- Ensure that the "onclick" handler is erased when a context closes.
- Do not clear the "onclick" handler in `contextMenus.update` if the
onclick key has been omitted (parity with Chrome).
- Remove some unnecessary `Promise.resolve()`
- Add extensive set of tests that check the behavior of the contextMenus
APIs with regards to the onclick attribute in various scenarios.
MozReview-Commit-ID: A5f3AUQzU8T
