move padding to extraData for padding support

We start to support Box model in QTBUG-41559, then we totally added 5 qreal and 4 bool like:

    qreal padding;
    qreal topPadding;
    qreal leftPadding;
    qreal rightPadding;
    qreal bottomPadding;
    bool explicitTopPadding;
    bool explicitLeftPadding;
    bool explicitRightPadding;
    bool explicitBottomPadding;

It could hurt performance a lot. We should move them into extraData.

Current fact:

• Text: most in extraData except padding
• TextEdit: all not in extraData yet
• TextInput: all not in extraData yet
• Positioner(Column, Row, Grid, Flow): all not in extraData yet
• Flickable: leftMargin/topMargin/rightMargin/bottomMargin are in AxisData now, special case.