demowindow: Use std::cerr instead of g_warning().
demowindow: Use std::cerr instead of g_warning().

Avoiding the need to use std::string::c_str(), which seems to be
far less necessary with C++11 now that we have functions such
as std::sto*(). This makes it easier to find c_str() in the source
code.

GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm