Cleanup blist popovers on close

Every time you right click on a blist node, a new popover is created. Since
it's attached to the widget, it 'leaks' until the blist is closed, and causes
several warnings when the widget is finalized.

Testing Done:
Right clicked on a few nodes, and then closed the buddy list. No warnings about extra children were raised. Also tried a few menu items to make sure they triggered Remove/Set Custom Icon/etc.

Reviewed at https://reviews.imfreedom.org/r/2046/