Add wxMessageDialog::GetEffectiveIcon() and use it in all ports.

Add wxMessageDialog::GetEffectiveIcon() and use it in all ports.

Remove code duplication and inconsistencies among different ports by using a
single function in the base class for the determination of the effective icon
style to use, correctly handling both wxICON_NONE and the absence of any
wxICON_XXX styles.

Closes #11822.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63725 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775