Add grid elements that can display external information

[klues]

For some use cases grid elements that can display information/states would make sense. For instance after implementing https://github.com/asterics/AsTeRICS-Grid/issues/74 it would make sense to have elements that can show e.g. a temperature of a Smart Home control application.

Ideas for implementation:

• add the possibility to add a boolean state (on/off, active/disabled) to every grid element. The state can be shown as little dot/bar within the element
• add own "state elements" which can show information of any type (boolean, number, text, ...)
• own menu/modal to define source of information. Sources could be:
  • result of a request to an URL/REST API, configurable update interval
  • internal AsTeRICS Grid states (e.g. play/pause state of YouTube player, current volume, current user, ...)
  • result of any action performed by an element. E.g. "Youtube Play" action can result in a promise which returns "playing" after the video has started. This result can e.g. be used to show as boolean state for the "play/pause" button.
• It should be possible to transform state values. For instance:
  • "playing" -> "true", any other value -> "false"
  • HTML response of a weather page -> current temperature as number

[klues]

See https://github.com/asterics/AsTeRICS-Grid/discussions/239#discussioncomment-6092665 for an idea showing time/date on an element. There should be possibilities to show date/time in a custom format.