tableDataView renders the nulls same way in _renderTable and _cellsDidChange
Refactored arrow image paths into constants for easier modification
Merge pull request #114 from kheldysh/constantified-popover-arrows

Refactored Flame.Popover's arrow image paths into constants for easier on-the-fly changing
Slight refactor, no change to logic
Merge pull request #113 from mattijauhiainen/rendering_nulls_tableDataView

tableDataView renders the nulls same way in _renderTable and _cellsDidChange