Can't select other xpage folder in picker for Page location (new page sidepanel)

Note it is an improvement as this was done by design

Problem

A channel can have more than one xpage (root) folder. In the New page sidepanel, the first one that is found is preselected as Page location for the new page. 

Expected: As a user you expect that you can use the picker to select another xpage (root) folder or its subfolders

Actual: The picker shows only the first xpage folder as root: you can only select that folder or one of its subfolders. There is no way to switch to another xpage 'root' folder

Result: You cannot create xpages in the other xpage folders and their subfolders (from the CM).

Requirements

In the picker for the Page location:

If there are more than one xpage folder in a root folder

• select the first xpage folder that is found in the content tree as default - show it in the side panel while creating pages so that user doesn’t have to select the location all the time

• if user click on picker to select location, show all folders in picker and only allow selecting xpage folders.
• allow users to select an available folder/subfolder in the picker
• If any other type of folder selected, deactivate the "OK" button.

Out of scope - for later

CMS-14322 When a different location is selected, the Document type dropdown in the sidepanel must be updated with the document types that are allowed in that folder.