wxWidgets 3.1+ accepts const argv arrays (const wchar_t* const* and
const char* const*) in wxExecute(), making the const_casts unnecessary.
Remove all 14 const_cast<char**>/const_cast<wchar_t**> wrappers around
wxExecute calls and their associated FIXME comments across GUI.cpp,
NotificationManager.cpp, and Downloader.cpp.