Fix behaviour of wxComboBox using ID=1000 in wxMSW.

If a wxComboBox was created with ID=1000, clicking in its drop down didn't
work.

It turns out that the native combobox control always creates its dropdown
listbox with the ID of 1000 and that when we were getting LBN_SELCHANGE from
it we mishandled this message because our code mistakenly believed that all
messages from the ID of the control itself were, in fact, coming from this
control, which wasn't the case here.

Fix this by adding a special check for this case in wxComboBox. Also
virtualize the dynamic cast which was done in wxWindow::FindItem() as this
makes the code simpler and allows to get rid of __WXUNIVERSAL__ #ifdef as
well.

Closes #15647.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75134 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775