Add a new board attribute 'minimizeReflow'

This new attribute lets the user choose if he wants to use the new reflow minimization technique. This is mainly for backwards compatibility because some usecases won't work with reflow minimization, e.g. buttons in texts with onclick handlers and looking up board elements by their DOM ids during updates. Default value is true, deactivate it by setting JXG.Options.board.minimizeReflow to false or adding this attribute to the attributes parameter object in initBoard().
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