Currently PagePool allows pages to be pushed onto the stack but there
is no way to pass initialization properties to the newly-created
page. This change adds support for this, and also provides
asynchronous object creation to speed things up a bit.

After running the following main.qml the page stack will first
push a page with the default title "NO PROPERTIES" because it
invokes the no-property behavior. Afterward the same page gets
pushed on the stack but this time it sets the title to
"PROPERTIES" using the passed-in properties object.

// main.qml
Kirigami.ApplicationWindow {

id: window

Kirigami.PagePool {
  id: pool
}

Kirigami.PagePoolAction {
  id: noprops

  text: "Action with props"
  pagePool: pool
  pageStack: window.pageStack
  page: "qrc:/TestPage.qml?hasProps=" + false
}
Kirigami.PagePoolAction {
  id: props

  text: "Action with props"
  pagePool: pool
  pageStack: window.pageStack
  page: "qrc:/TestPage.qml?hasProps=" + true
  properties: { title: "PROPERTIES" }
}

Component.onCompleted: {
  noprops.trigger()
  props.trigger()
}

}

// TestPage.qml
Kirigami.Page {

title: "NO PROPERTIES"

}