Close Notification Window after a certain time or after losing intial focus.

Original Title: MetaMask sometimes prevents taskbar auto-hide

**Describe the bug**
In some cases, especially where a MetaMask notification popup is delayed in coming by more than the time it takes for a user to switch focus to another program, MetaMask prevents that user's Windows Taskbar from auto-hiding, which sometimes requires a search through all open windows to identify what is holding the taskbar unhidden.

**To Reproduce**
Note: These are not 100% reliable, but work some percentage of the time. If you don't see the "Actual Behavior," repeat steps 9-12 but try doing step 11 faster.
1. Set Windows Taskbar to be in a vertical position on the right or left of the screen.
2. Set Windows Taskbar never combine buttons and (optionally?) use small buttons.
3. Open so many windows that they continue to a second column, with a horizontal scroll bar on the taskbar.
4. Set Windows Taskbar to auto-hide.
5. Open a maximized window of a non-browser program on the screen where you will bring up a smaller browser window.
6. Open a smaller browser window with MetaMask, and log in.
7. Go to Settings, Security & Privacy, click Clear Privacy Data, and Confirm.
8. Visit a dapp page that requests account information on load, maybe after a brief delay.
9. In the connection request popup, click Cancel.
10. Reload the page (F5) or re-trigger the connection request.
11. Before the page has loaded or the connection request is triggered, quickly click on the other non-browser program to give that focus and have its display take over the screen. This might even be on a different monitor, in which case the browser might still be visible but not in the user’s field of active view.
12. A Connection Request window will pop up, maybe in a place where a user doesn’t see it or it gets buried beneath other content from other windows.

**ACTUAL BEHAVIOR:**
13. The taskbar stops auto-hiding.
14. If the user wishes to figure out which program is causing the taskbar to not auto-hide, it may be a somewhat long guess-and-check search through open windows. If the user's current program is in a different "page" / column of taskbar items than the one with the MetaMask notification, there is nothing visible about why the taskbar is held open.
15. At the end of the list of open windows, there is a taskbar titles “MetaMask notification.”
16. After clicking into that window, whether clicking Cancel or Connect or neither, the taskbar returns to autohiding successfully. Clicking in the browser window also works.

**EXPECTED BEHAVIOR:**
13. MetaMask does not prevent the taskbar from autohiding. If the user has left a notification window without reply, the user should be able to see the box with a number on the MetaMask logo after returning to the dapp, and click the MetaMask logo to bring up the notification again, and respond to it.

**Browser details (please complete the following information):**
- OS: Windows 10 Pro
- Browser Chrome 72.0.3626.121 (Official Build) (64-bit)
- MetaMask Version 6.3.1

Start by reproducing the Windows 10 taskbar behavior using the numbered steps, focusing on a delayed MetaMask connection request after the browser loses focus. Done means an unanswered notification no longer prevents taskbar auto-hide and remains available through the MetaMask badge for later response.

Written by the indexing model from the issue text.