Bug 971577 - [System] Make visibility manager instantiable
Merge pull request #16591 from rexboy7/system-bug971577-bubbletea

Bug 971577 - [System] Make visibility manager instantiable. r=alive
Bug 961800 - Implement Child Window Factory
Merge pull request #16556 from alivedise/bugzilla/961800_master/child-window-factory

Bug 961800 - Child Window Factory, r=etienne.
(WIP) Bug 971498 - [System2] Instantiable HomescreenLauncher
Address Alive's comment
Add jsdoc and resolve conflict