Add back button handling in Sample

[maxkoshevoi]

Sample has many things disabled for setting up if the main package works. removing legacy code changing code to make better sense, in my opinion, however this may lead to more bugs.### :sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

:arrow_heading_down: What is the current behavior?

Popups in Sample don't react on back button

:new: What is the new behavior (if this is a feature change)?

• Sample now handles back button
• Moved Essentials.Init to MainApplication to not clatter MauiProgram with #ifs, and I hope it won't be needed when MAUI releases anyway. Totally subjective opinion, Init can be moved back in MauiProgram

:boom: Does this PR introduce a breaking change?

No

:bug: Recommendations for testing

:memo: Links to relevant issues/docs

684

:thinking: Checklist before submitting

• [x] All projects build
• [x] Follows style guide lines
• [ ] Relevant documentation was updated
• [x] Rebased onto current develop

[LuckyDucko]

Init makes more sense in MainApplication for now, solely as its less to ask people to add in, as well as its not directly relevant to them.

We might push it back/add ome 'override' to MauiProgram later if people need it.