Check box column multiple fixes

Fixes the following problems:
* Subclassing the QCheckboxColumn results in constructor errors
* Calling GetColumnByName on a datagrid with a QCheckboxColumn pretty much blows up because the checkbox column overrode the ->Name attribute to output its HTML.
* The QCheckboxColumn example page was broken by the bugfix branch. The backticks were replaced with double-quotes around column names in order to support Postgresql. However, MYSQL only supports double-quotes around column names in ANSI mode, or it thinks you are trying to quote a literal string. The quotes are only necessary if a column name is also a reserved word, so I just removed them. I trust all SQLs support this.