Optimize selection peformance
* _Base.js:_highlight() line 186, avoid highlighting rows which is out of the body
* _Base.js:_highlight() line 178, make sure do highlighting once on one item
* Row.js:_onMark line 356, avoid query rows which is out of the body
* body:_buildRowContent(), _buildRows(), save renderred row ids.
* body:refresh(), clear saved rednerred row ids.
Merge pull request #258 from shanghaikid/optimizeSelection

Optimize selection peformance