Popper: Fix memory leak on route change (#7715)

[a-kriya]

Fixes #7715 by addressing the two following issues:

• When the route changed on click, the doDestroy method of vue-popper was returning right away without actually destroying the popper due to showPopper being true. forceDestroy parameter is added to handle this case. An alternative fix could be to set showPopper to false in beforeDestroy before calling doDestroy, but the parameter approach was chosen as it can be reused.
• After the above fix, the destroy method of popper.js was being successfully called, which in turn called _removeEventListeners. The target that this method was removing the scroll event listener from was not the same element to which the listener was added (this is due to getScrollParent not being able to find the correct element as some of the top level nodes had already been destroyed). This caused the event listener (bound to this) to leak, and consequently the whole associated DOM and VDOM tree. The fix is to replace the logic to find the target in _removeEventListeners and use the stored target to remove the listener from.

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[a-kriya]

Second issue:

With the fix: