Fix positioning issues and change content y repositioning behaviour

Refactor the code for re-positioning the visibleItems.first() in
QSGItemView::applyModelChanges() and fix various positioning issues.

The positioning behaviour for removing items at the start of the view
has  changed. This action will no longer cause the content y to move
down; previously the content y would always move down unless another
item was moving backwards to the first visible position. This will make
it easier to implement built-in removal transitions for the views, since
removed items cannot be animated if the content y jumps down past them.

Additionally, moving items from before a GridView should not cause the
top item in the view to move upwards and out of the view.

This also adds additional remove tests and fixes the
tst_QQuickGridView::moved() test which wasn't waiting for the polish
event after setting the content y, which caused some of the tests to
pass even though their test data was wrong.

Change-Id: Idad11a73a18f88208e73a52111ed839458e05f2d
Reviewed-by: Martin Jones <martin.jones@nokia.com>

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