Add TooltipManager

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Add support for keyBindingTarget to TooltipManager::add
:pencil: Document TooltipManager
Return a Disposable from TooltipManager::add
Remove ability to add tooltip with selector

It’s incompatible with returning a disposable because there’s no way
to distinguish multiple tooltips added to the body with different
selectors. Maybe someday.
Add atom.tooltips global
:pencil: returned disposable
Merge pull request #4250 from atom/ns-tooltip-manager

Replace jQuery-based tooltip API with atom.tooltips global