terminalpanel.cpp *should* compile on Windows, so there is no reason to
remove it from the build and use tons of #ifdefs in dolphinmainwindow.
We still keep the terminal panel disabled on Windows
(i.e. the two remaining #ifndef Q_OS_WIN in dolphinmainwindow),
because it is probably not functional.
But at least we won't break the Windows CI every time
someone touches the terminal panel code (see e.g. 2e942237c9).