[Enhancement] Add Loaded event for page

[jingliancui]

Summary

ContentPage now just has OnApearing override method and the Appearing event , but doesn't has Loaded event .
Then now meet an "issue" , maybe , do navigation to the secondpage in the mainpage OnApearing method , result is I can not return to mainpage after I press the back button on the secondpage .
Then search on the internet and I found this https://forums.xamarin.com/discussion/22561/page-loaded-event .
Use the SizeChanged event as the "Loaded" event .
But I don't know if it is can use as "Loaded" event .

API Changes

Page class add Loaded and UnLoaded Event .

Intended Use Case

Page class add Loaded and UnLoaded Event .

[PureWeen]

duplicate #30