fix(tooltip): clean up stackedMap on scope destroy

[RobJacobs]

The tooltip was not being removed from the stackedMap
resulting in a memeory leak. Tooltip will now be
removed from the stackedMap in scope destroy
function.

Fixes #4604

[wesleycho]

LGTM

[Foxandxss]

Please, add tests.

[wesleycho]

Not sure we can add a test here, the stacked map is not exposed publicly.

[Foxandxss]

You inject the stacked map in the test, you add a few tooltips, you destroy one, you check that the stack has one tooltip less.

[wesleycho]

But the stack that needs to be checked is the internal one - that is the one that gets removed

[Foxandxss]

@RobJacobs tell us your secrets to close a PR as merged like you did without creating extra unwanted history messages.

[RobJacobs]

@Foxandxss When working in a PR branch, after I make the initial commit I always use 'git commit --amend' to keep my commit history clean. Then when I merge to master, rebase the PR branch from master, then in the bootstrap repository:

git checkout master
git pull <fork-url> <branch>
git push origin