Mac: Fix crash in dw_window_set_font() with NULL font name. Not sure if I am getting the default font correctly, may need to revisit. Also switched Helv to Helvetica so it works on Mac, might need an #ifdef if this doesn't work on other platforms.

[Desktop Entry]
Name=Dynamic Windows Test
Exec=@prefix@/bin/dwtest 
Terminal=false
MultipleArgs=false
Type=Application
Categories=Application;Development;X-Red-Hat-Base;