xmacro: declare in header files, instantiate instead of include

This works better with meson, where .h files can be declared as being part of an
executable easily, but I couldn’t find a way to declare
e.g. include/atoms.xmacro as a dependency.

related to #4086

i3-config-wizard/xcb.h

@@ -3,6 +3,8 @@
 /* from X11/keysymdef.h */
 #define XCB_NUM_LOCK 0xff7f
 
+#include "i3-config-wizard-atoms.xmacro.h"
+
 #define xmacro(atom) xcb_atom_t A_##atom;
-#include "atoms.xmacro"
+CONFIG_WIZARD_ATOMS_XMACRO
 #undef xmacro