qml: improve batch rendering of GridView

Items in the grid view overlaps each other due to their shadow effect, by grouping them every even index (vertically and horizontally), we can ensure that items with the same Z index won't overlaps. This allows items of the same group to be batch rendered by the engine.

before:

every shadow is rendered in its own batch

after

shadows are rendered by group (orange, magenta, red, raspberry)