View.Add/Remove should throw exceptions if invalid

[tig]

TileView tests assume Views can be Added to SuperView twice

e.g.:

[Fact]
public void TestDisposal_NoEarlyDisposalsOfUsersViews_DuringInsertTile ()
{
    TileView tv = GetTileView (20, 10);

    var myReusableView = new DisposeCounter ();

    // I want my view in the first tile
    tv.Tiles.ElementAt (0).ContentView.Add (myReusableView);
    Assert.Equal (0, myReusableView.DisposalCount);

    // I've changed my mind, I want 3 tiles now
    tv.InsertTile (0);
    tv.InsertTile (2);

    // but I still want my view in the first tile
    // BUGBUG: Adding a view twice is not legit
    tv.Tiles.ElementAt (0).ContentView.Add (myReusableView);

    Assert.Multiple (
                     () => Assert.Equal (0, myReusableView.DisposalCount),
                     () =>
                     {
                         tv.Dispose ();
                         Assert.True (myReusableView.DisposalCount >= 1);
                     }
                    );
}

[tznind]

Should have a remove call first before adding to the other view.

Test doesn't want view in 2 places at once, at most it wants to see view move to new home.

It can be either

• when you add view to second place, it does implicit Remove
• or, when you add to second place it throws Exception

Either works for me

[tig]

The View Disposal tests also have a similar problem, but they add a view to multiple superviews.

I'd like to have this fixed after I merge

• Cleans up/Refactors View.Subviews  #3962

AND, to make View.Add/Remove throw exceptions when these situations are detected. The both emit warnings now, but I had to remove the throws for tests to pass.