The Wayland backend was already not supporting this setting
since it is an XSetting that is not backed by a GSetting.
Drop this setting altogether, since we will stop supporting
general-purpose modules.
- add gtkmodulesprivate.h and move stuff there from gtkprivate.h
- add gtkprivate.c and move stuff there from gtkmain.c
- add gtkwin32.c and move stuff there from gtkmain.c
- don't redefine GTK_DATADIR and friends in gtkprivate.h
- have _gtk_get_datadir() and friends on all platforms
- remove the horrid hacks where gtkprivate.h can't be included,
or must be included later due to redefinition of the compile-time
directories
