This undocumented "private" class was used for various windows UxTheme
functions which are available since WinXP. As wxWidgets 3.1 is XP+ it
does not make sense anymore to load the theme functions dynamically.
wxSystemThemedControl allows to use the "system theme" (i.e. the theme used by
the system applications such as file manager and which can, surprisingly, be
different from the default one). Currently it is only implemented for wxMSW
and does nothing under the other platforms.
Use wxSystemThemedControl for wxDataViewCtrl, wxListCtrl and, optionally, if
wxTR_TWIST_BUTTONS style is specified, wxTreeCtrl to give them more native
appearance under MSW.
Closes#16414.
