Fix for bug #6943, improvements in io.request.AbstractRequest, io.rest.Resource, logging, ...

- fixed [bug #6943](http://bugzilla.qooxdoo.org/show_bug.cgi?id=6943)
- added property `parseErrorResponse` and response parsing for requests with erroneous status in `qx.io.request.AbstractRequest`
- added properties `requestHeaders` and `requestProperties` in `qx.io.rest.Resource` for convenience
- added property `serializeLogArgs` in `qx.log.Logger`
- added apply method for property `required` in `qx.ui.form.MForm` (this simplifies property usage in classes)
- added customization of row renderer in `qx.ui.table.Table` constructor
- added property `fontSelector` in `qx.ui.table.rowrenderer.Default`
- added some helper methods in `qx.ui.treevirtual.SimpleTreeDataCellRenderer`; refactored method `_addImage`
- added helper method `specToCode` into `qx.bom.Html`
- improvements in `qx.event.Timer`
- fixed padding calculation for images inside table cells
- added ability to output log items as is in `qx.log.appender.Native`
- fixed a typo in generator docs