[Feature Request]: Create a print method for the teal_app object

[vedhav]

Feature description

The app object created using the init function is a list of UI and Server functions. It will be nice to have a print method for this object.

Proposal:

TEAL APPLICATION
  |-UI
  |  |-Title: teal app title (without favicon)
  |  |-Header: Show this row if the header is set.
  |  L-Footer: Show this row if the footer is set.
  L-Server
     |-Data: list of objects from the `teal_data` object
     L-Modules: Label of the modules

Please also write a unit test for the format method (refer to the module's format function).

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