this involves some changes to the workflow defaults:
* workflow.start_default() takes no arguments
* workflow.set_default() (originally replace_default) configures the
default that will be started by next call to start_default().
The intended usecase is to set_default() first and then start it
separately.
* apps.base.set_homescreen() factors out the logic originally in
main.py, that decides which homescreen should be launched. This uses
set_default() call. start_default() is then used explicitly in main.py
