Only click when pointer is over the button.

Previously a button click was carried out when `:pointerover` was set, but because the button captures mouse input on mouse down, this is always true while the mouse is held. Check that the pointer was released within the bounds of the control.

Fixes #938

src/Avalonia.Controls/Button.cs

@@ -234,7 +234,7 @@ namespace Avalonia.Controls
             PseudoClasses.Remove(":pressed");
             e.Handled = true;
 
-            if (ClickMode == ClickMode.Release && Classes.Contains(":pointerover"))
+            if (ClickMode == ClickMode.Release && new Rect(Bounds.Size).Contains(e.GetPosition(this)))
             {
                 RaiseClickEvent();
             }