UX: Allow usage of sub-folders under Mission (Android/iOS)

[Saijin-Naib]

Tell us a bit about the feature:

• What problem does it solve?
  • Clutter/focus when planning extensive surveys
• What does it do?
  • Provides an intuitive/hierarchial means to organize plans
• What flight stacks must it work with? (All, PX4, ArduPilot)
  • All
• Relevant vehicle types? (All, multirotor, fixed-wing, VTOL, submarine, etc.)
  • All
• Are there other systems that have this feature?
  • Sort of. Solex/Tower/MP use system file open dialog
• What communication/integration standards does the feature rely on (e.g. MAVLink commands etc.)
  • N/A
• Any additional context you can provide will make the feature easier to evaluate (e.g. mockups, detailed specification, etc.)
  • N/A

[Antiheavy]

+1 for this request. FYI @dakejahl