Keep order of scripts added to QWebEngineScriptCollection

Use a QList instead of a QSet to store the scripts in the collection.
This avoids situations where two scripts injected depend on each other,
and fail or succeed depending on the semi-random order that QSet imposes.

Change-Id: I44d5d89866ff2431544cc91afb1c102d93daa5da
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>