Add Xamarin.Forms version

[nirinchev]

TODO:

• [x] Styling
• [x] Decide on editing mechanism (check how Android does it)
• [x] Logout functionality
• [x] Ignore the resources file.
• [x] Entry styling
• [x] Behavior for selected item
• [x] Highlighting on iOS styling
• [x] Completed list styling

Potential limitations:

• [ ] Ordering. XF does not yet support reordering listviews, so potentially a custom renderer will be needed.
• [ ] Swipe gestures on Android - it seems XF by default uses the long press gesture on Android for context items.
• [ ] Separators are always visible on iOS - looks like a XF bug.

Outstanding issues:

• [ ] Realm needs to be passed. Getting it on the main thread in each VM will run into caching issues - https://github.com/realm/realm-dotnet/issues/933
• [x] Count is not updated. https://github.com/realm/realm-dotnet/issues/979
• [x] Annoying workaround for add item when the list is empty. https://github.com/realm/realm-dotnet/issues/978
• [x] Occasional crashes when navigating back-and-forth and then editing title https://github.com/realm/realm-dotnet/issues/980
• [x] Setting of primary key is incorrect. https://github.com/realm/realm-dotnet/issues/1002
• [x] Crashes when adding new items due to https://github.com/realm/realm-dotnet/issues/991