view-stack: add AdwViewStackPages:selected-page property
This adds a property to AdwViewStackPages that can be notified containing the visible-child's AdwViewStackPage.
What makes this useful is in data-binding in UI files for information about a visible child such as page title or icon-name.
For example, this allows something like:
<object class="AdwWindowTitle">
<binding name="title">
<lookup name="title" type="AdwViewStackPage">
<lookup name="selected-page" type="AdwViewStackPages">
<lookup name="pages">stack</lookup>
</lookup>
</lookup>
</binding>
</object>
Edited by Christian Hergert