XF 4.x support

[jdluzen]

Hello and thanks for this great project!

After directly using the nuget package in my project, neither the BeforeAfter.BeforeView nor BeforeAfter.AfterView showed up.

I ran your sample app which ran successfully on XF 3.3. I then updated the packages to XF 4.4, and the Before/AfterView stopped showing up.

[jdluzen]

On XF 3.3, OnSizeAllocated is called with what looks like proper values, ~314 in my case.
On XF 4.4 OnSizeAllocated is called with 1, 1.

[jdluzen]

I've spent a good amount of time debugging this, and it seems to be isolated to the ContentPresenter. The view isn't being rendered at all. I'm unsure what has changed since XF 3.3.

[gsgou]

@jdluzen, @mariusmuntean did you by chance find any workaround?
Just noticed the issue persists also on 5.0.0.2515.

[gsgou]

@jdluzen, @mariusmuntean send a PR with a fix.

The ContentPresenter should be placed in a control template to mark where content to be displayed by the templated custom control will appear. I replaced it with a ContentView.

https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/templates/control-template#substitute-content-into-a-contentpresenter