Generate keys during modeler feature save

As a Studio app builder, translations keys are generated when I save

• a doctype: keys for schema properties (any schema associated to the doctype)
• a schema: keys for properties
• a page provider: keys for predicates and aggregates
• a workflow:
  • keys for workflow variables
  • keys for node variables

AC

• Translations keys are created in all existing json files in Designer
  • If no file exists yet, they are added to the messages.json file only
• Translations keys are generated only when they don't exist in the file already
• Keys created respect the convention described in: https://docs.google.com/spreadsheets/d/1A3PP9uLI5ErH7BPO3xXiXPmDX2SH_FmCCSyzXTfuBcY/edit#gid=0&fvid=1155444772