Rearrange PdfPageView.qml

Now it looks more similar to PdfMultiPageView.qml: public properties
and functions are grouped by functionality, implementation details are
"below the fold", and properties and functions that we don't want to
expose as API are nested inside inner items.

Also fixed ColumnLayout attached properties.

Change-Id: Iafe6f983a34ca6bac9b0d4f3a26aba5a426e0232
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>