Improvements on the Planned Downtime section

* pandora_console/godmode/agentes/planned_downtime.editor.php: Fixed an error
	which made impossible the creation of new periodic downtimes.

* pandora_console/godmode/agentes/planned_downtime.export_csv.php: Added file.
	Exports the planned downtimes list to a CSV file.

* pandora_console/godmode/agentes/planned_downtime.list.php: Added a new form
	to filter the list. Added pagination. Changed the items order. Error fixes.