[Request] System command type in Controls structure

Related to closed case #1095. I believe it's worthy of another look.

Currently, to create custom system commands a user must use the Event or System structures of config.yaml.

In my specific case, I would like to send a parameter to the system for use with RPi-gpio applications. This would be most intuitively accomplished through the controls structure with a slider or value-entry field.

In the Events structure, the command type is configurable between system and printer/g-code types. Rather than use a Controls command to send a "dummy" printer command (M300 or other) to trigger an event to send a system command, adding the "type" switch to the Controls structure would allow the script to be sent directly from the Controls command without limiting the user to the predefined Event list or the static nature of the Systems tab.

controls: ..-.children: ....name: xyz ....width: '2' ....type: system <-------------------------------------- new option ....command: gpio abcParameter ....input: ....-.default: 10 ......name: abcParameter ......parameter: abcParameter