Panels: Correctly set panel classes on pages with external panels

This is an update to #6658

When using external panels, classes set on a page/content wrapper are 
set inside `getPage` with:

`this._parentPage || $( "." + $.mobile.activePageClass )`

If a transition to a new page is triggered from an open, **external** panel, 
**active page changes while the panel plugin still updates element 
classes**, which results in part of the classes being assigned to the page 
being left and part of the classes being set on the new page (log active 
page id inside `getPage` to see).

This PR adds a new parameter `_openedPage`, which if set overrides both 
parent and active page and points to the correct page in need of panel-related 
updating.

Fixes: #6650
Closes: #6658