[AdaptivePageLayout] Add signal for when a page is added

[Michael Hall]

Add a signal for when a page is added to the AdaptivePageLayout that incudes the page and column index as parameters

Suggestion:

pageAdded(page, columnIndex)

Use Case:

In uReadIt I want to respond to an ArticlePage being added to the 2nd column by adding a CommentsPage to the 3rd column (only when in a 3+ column layout). But that must be done *after* the ArticlePage is added to the AdaptivePageLayout. Both the ArticlePage.onCompleted callback, or the incubator's onStatusChanged to Component.Ready are called *before* the page is added to the AdaptivePageLayout.

[Zsombor Egri (zsombi)]

Right. We can add this, however will always report the column number even if the layout has 1 column.

Example:
apl.addPageToNextColumn(primaryPage, page) -> pageAdded(col2Page, 1)
apl.addPageToNextColumn(col2Page, page) ->pageAdded(col3Page, 2)

This will happen even if the layout is 1 or two columns! Perhaps an additional parameter containing the effective column can be added later if needed.