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Fix nextTick tigger before really render

• Add key for rendering items #220
• Support prop itemScopedSlots #219

Props dataKey not allow to number.

Fix props type warning #211

Fix estimateSize will be used as a fallback #207

• Fixed page mode bugs #204 #205

• Support updatePageModeFront public method

Fix average size calc error #202

Delete size map when items reducing

Support page mode #199

Support style for item, wrapper, header and footer.