Add support for fine grained page.xml (now we have support for coarse grained pages.xml) in the graphical pages.xml editor.

I imagine an additonal tab or a toggle on the current tab that when enabled shows not just the pages that are in the single file, but also includes any info from other *.page.xml files.

In the first iteration we can just make this a read only mode for any external files and then double click for editing (maybe that will be too cumbersome for the users ?)