Ensured ComboBox is not closed when scrolling
Ensured ComboBox is not closed when scrolling

Resolves issue #471
tipunch74/MaterialDesignInXamlToolkit,tipunch74/MaterialDesignInXamlToolkit,ButchersBoy/MaterialDesignInXamlToolkit,ButchersBoy/MaterialDesignInXamlToolkit,tipunch74/MaterialDesignInXamlToolkit,ButchersBoy/MaterialDesignInXamlToolkit,DingpingZhang/MaterialDesignInXamlToolkit