Berry zigbee.started()

[s-hadinger]

Description:

Added zigbee.started() -> bool or nil:

• returns true if zigbee sucessfully started, then all other zigbee methods are available. This state is final and does not change.
• returns false if zigbee is still in initialization process. This state eventually changes to true or nil
• returns nil if zigbee is not configured (no GPIO) or if initialization failes. This state is final and indicates a fatal error.

Other change:

• Zigbee buttons are not displayed on front page if initialization failed

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.7
• The code change is tested and works with Tasmota core ESP32 V.3.0.4
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass