Rrd fixes

These patches improve the RRD functionality:
* Fix [not possible anymore to create new graphs](https://www.domotiga.nl/issues/480) by properly quoting strings in FEditGraphs.ButtonGraphSave_Click (commit ce5ac5e9f7c8849b4ab89a4faccbd9bff4b3d424)
* Add more Client-Server functionality (JSON rrdtool.generate and rrdtool.delete)
* Pop up error if something is wrong and more debug logging
* Other minor fixes

See the individual commits for details.