Only reload the popover in Safari, when itself is shown for a different tab

I mistakenly assumed that there can only be one popover. And added an event listener that is called when any popover is shown, to all popovers, in order to reload the popover. However each browser window has its own instance of the popover/bubble. So this patch restricts the the event listener to the current popover.

[Wladimir Palant, 2014-01-13 12:39:28]

LGTM

[Sebastian Noack, 2014-01-15 17:57:12]

I have uploaded a new patchset, that also reloads the bubble only when it is
shown for a different tab as before. There is no point in reloading the popover
when it is supposed to show the same data anyway. The javascript for the bubble
already takes care of keeping its contents synced for the tab that was active
while it was loaded.

[Wladimir Palant, 2014-01-16 08:43:05]

Still LGTM