input-widgets: fix a bunch of bugs

[ES-Alexander]

• Listen to Data Lake variable IDs instead of names
  • Fixes #1871
• Stop listening to the old variable when switching to a new one
  • Includes removing an excess listener that was being created as part of initialisation (now it only creates the onMounted one, not the watch one as well)
• Initialise values correctly
  • Set to the current data lake variable value when a variable is specified
  • Set to the last widget value if no variable is specified on startup
• Allow resetting the linked variable after reopening the edit pane and on startup
• Make new variables persistent, so they can be edited and deleted from the data lake table