Simplify the process to execute a system task

Tasks in form of behavior trees can be assigned to a system assembly (see Task Modeling).

The current way to execute a task after the system sequencer is launched involves tedious interactions with the CLI, see, e.g., Step 2 in the Nav2 Demo.

This issue is to simplify the process and includes the following contributions:

• Code Generator
  • change the install directive of system package's CMakeLists.txt, to include the BT xml model of first task in the system task list
  • add default_bt_xml_filename to param.yaml that refers the BT xml model above
  • add a directive in the system sequencer launcher to read a parameter file with substitutions, to correctly interpret the path to the BT xml
  • add config & activate transitions for the system sequencer activate.py
• UI
  • add a new menu item visible when the system sequencer is active and that sends the bt_sequencer_msgs/action/ExecuteBt action goal