Fix homescreen example thread safety. (#431)

This addresses and fix:
https://github.com/ArthurSonzogni/FTXUI/issues/430

This patchs makes |shift| to be read and written on the same thread. We
request the update to be made via a task posted on the main thread.

This patch has no real consequence, the previous behavior was fine.
I hope it will help users not to have thread safety issue and better
understand they can post tasks this way.