fix(tooltip): remove tooltips when element destroyed with event='hover'

[doug-a-brunner]

I have a list of recent files in the Electron version of my app that changes when the contents of the file system do, and I found that when a file disappeared at the same time that I had a hover tooltip visible for it, that tooltip got stuck onscreen. This appears to fix the problem. I tried to create a unit test for it but found some very strange behavior:

1. The setTimeout functions used on the existing tests are never called. It seems as though they should be using Jasmine's done() mechanism to force the test to wait for the timeout function.
2. When I tried doing this, or creating a test for hover by calling timeout.onMouseEnter() and timeout.onMouseLeave(), calling expect(tooltip.tooltipElement).toBeUndefined() caused the browser to hang.

[danielsogl]

please rebase on master