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

include/i3-atoms_rest.xmacro.h

@@ -0,0 +1,22 @@
+// clang-format off
+#define I3_REST_ATOMS_XMACRO \
+xmacro(_NET_WM_USER_TIME) \
+xmacro(_NET_STARTUP_ID) \
+xmacro(_NET_WORKAREA) \
+xmacro(WM_PROTOCOLS) \
+xmacro(WM_DELETE_WINDOW) \
+xmacro(UTF8_STRING) \
+xmacro(WM_STATE) \
+xmacro(WM_CLIENT_LEADER) \
+xmacro(WM_TAKE_FOCUS) \
+xmacro(WM_WINDOW_ROLE) \
+xmacro(I3_SOCKET_PATH) \
+xmacro(I3_CONFIG_PATH) \
+xmacro(I3_SYNC) \
+xmacro(I3_SHMLOG_PATH) \
+xmacro(I3_PID) \
+xmacro(I3_FLOATING_WINDOW) \
+xmacro(_NET_REQUEST_FRAME_EXTENTS) \
+xmacro(_NET_FRAME_EXTENTS) \
+xmacro(_MOTIF_WM_HINTS) \
+xmacro(WM_CHANGE_STATE)