Bug 1615588 - Extended nsIPromptService to support tab modal prompts. r=johannh,MattN,necko-reviewers,dragana

This patch introduces a new tab modal system prompt type. It can be opened via the nsIPromptService
with a destination BrowsingContext. These tab system prompts overlap slightly with the upper
chrome UI to differentiate them from content prompts (previously called tab prompts).

- Extended nsIPromptService and nsIPrompt to accept 3 types of modal prompts:
  - Window prompts
  - Tab (system) prompts
  - Content prompts (the old tab prompts)
- Removed prompt code from Prompter.jsm, always call PromptParent window actor instead
- Added PromptChild window actor to forward pagehide events to parent actor
- Created additional prompt methods in nsIPromptService to prompt by browsingContext and modalType
- Backwards compatibility is maintained, consumers can still open content prompts calling nsIPrompt with a content window

Differential Revision: https://phabricator.services.mozilla.com/D66446