[qtdeclarative] Add displayMarginBeginning/End to ListView and GridView

It is common to need to show content outside the visible area of
the view.
Merge pull request #25 from rburchell/master

Delay calculation of offline storage path as long as is practically poss...
Merge pull request #17 from martinjones/displayMargin

[qtdeclarative] Add displayMarginBeginning/End to ListView and GridView
Improve memory usage of QQuickTextNode.

The typical number of BinaryTreeNodes needed for a text line should be
much lower than 256.

Task-number: QTBUG-32770
Change-Id: I85aa161eb7cb6e55657213304b7577a0a33f1b67
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>