other than CBS_SIMPLE, parent of listbox is set to desktop.
- In CBDropDown. ComboBox uses only first item to calculate height of
  dropped listbox. Also if listbox is empty its height is unmodified
  (previously it was set to 0).
- Added correct handling of WM_GETDLGCODE and WM_(SYS)KEYDOWN messages.
- General clean-up. Message order is now more precise (at least
  notifications to client); listbox - combobox interaction has slight
  differences comparing to Windows.