Replace the current static page managing by object incubator

[Gustavo Pichorim Boiko]

Depending on the contents of the page, its asynchronous creation might take a really long time because of the internal behavior (or bug?) of QtQuick.
Loading pages synchronously fixes the problem, but the way it is implemented in messaging-app, there is a risk of leaking memory due to pages not being deleted properly when switching views.

There is another way to workaround this QtQuick limitation which is to force the page creation to finish before returning to the event loop, in the AdaptivePageLayout context, it would look like this:

var incubator = layout.addPageToNextColumn(originalPage, newPage, properties)
incubator.forceCompletion()

In order for this to be fixed bug #1544745 in AdaptivePageLayout first needs to be fixed.