Make background page objects aware of page variables

This example shows a techical drawing with a data field.
This is my literal use case. It utilizes the global standard placeholders (e. g. %pagenumber%).

However, I often find usecases where I want to add additional fields on a per page basis like e. g. a document status (draft, final, etc.) or author. This is similar to feature request #480 which asks for document-wide placeholders which could be useful, too but are not a necessity for background pages as in this case static text works just fine.

The example given in #480 could work wonderfully, if only background page objects could use page variables.

Example

• A text field on a background page uses variable %author%.
• On the background page the variable is set to Alice.
• Each page will now show the name Alice.
• Some pages however may define a different author. Still using the background page object but overwriting the placeholder value.

Start with the background-page behavior described in the linked example and review feature request #480 for the related placeholder semantics. Trace how page variables are defined and resolved for background page objects; done means a background value is inherited by each page and can be overridden on individual pages.