D5416: Polish ItemViewHeader, solve binding loops

[Aleix Pol Gonzalez]

apol created this revision.
Restricted Application added a project: Kirigami.
Restricted Application added a subscriber: plasma-devel.

REVISION SUMMARY
  Don't assume "page" in the namespace of ItemViewHeader, use view instead
  which we have and symbolizes the same.
  Don't change the implicitHeight when contentY is < 0.
  
  Smooth ItemViewHeader
  
  Make page property readonly. Nobody is accessing it and it's private
  anyway.
  When there's an image, don't wait to have the image to resize the header,
  it makes the whole UI change without much sense as it should be fast.
  Properly access the page property by name of the object, rather than having
  QML go through the object tree. It failed to find the property sometimes.

TEST PLAN
  Tests still pass, I don't get tons of warnings on Discover

REPOSITORY
  R169 Kirigami

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D5416

AFFECTED FILES
  src/controls/AbstractItemViewHeader.qml
  src/controls/ItemViewHeader.qml

To: apol, #kirigami, leinir, mart
Cc: plasma-devel, apol
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