merge row-height-799-800-801.  author: moe, reviewer: exarkun.  fixes js bug with uninitialized _rowHeight instance variable of ScrollingWidget.  closes #799, #800, #801, #813
merge row-height-799-800-801.  author: moe, reviewer: exarkun.  fixes js bug with uninitialized _rowHeight instance variable of ScrollingWidget.  closes #799, #800, #801, #813

twisted/quotient,twisted/quotient