implement a table of atoms defined only once

Previously, adding a new atom into the code required:
1. In <icc.h>, add the line `extern Atom my_new_atom;` declaring the new atom.
2. In <icc.c>, add the line `Atom my_new_atom;` defining the new atom.
3. In <icc.c>, add the line `my_new_atom = XInternAtom(dpy, "MY_NEW_ATOM", False);`
   interning this new atom, and assigning the interned value to the variable.

Now, just add a line with the atom's name in the X-MACRO in <icc.h>, and
a XInternAtoms(3) and a set of macros will do the rest.

All new atoms will be referred to as an entry in the ATOMS[] array.
For example:

	ATOMS[MY_NEW_ATOM]

The exception are for those REALLY OLD atoms that have already a
dedicated compile-time-known value defined at <X11/Xatom.h>.  Those
do not need to be interned; and are referred to with a constant
beginning with the `XA_` prefix.  For example:

	XA_WM_CLASS

icc.h

@@ -17,15 +17,12 @@ extern long _getprop(Window, Atom, Atom, long, char **);
 extern void getwflags(Client *);
 extern Window get_transient_for(Window);
 
-extern Atom wm_state, wm_change_state, wm_protocols, wm_delete, wm_take_focus, wm_colormaps, wm_hints, amiwm_screen, swm_vroot;
-
 #define Pdelete 1
 #define Ptakefocus 2
 #define Psizebottom 4
 #define Psizeright 8
 #define Psizetrans 16
 
-
 #define mkcmessage(w, a, x, ...) (&(XEvent){.xclient = { \
   .type = ClientMessage, \
   .window = (w), \
@@ -34,4 +31,33 @@ extern Atom wm_state, wm_change_state, wm_protocols, wm_delete, wm_take_focus, w
   .data.l = {(long)(x), CurrentTime, __VA_ARGS__}, \
 }})
 
+#define ATOMS_TABLE(X) \
+  X(AMIWM_APPICONMSG) \
+  X(AMIWM_APPWINDOWMSG) \
+  X(AMIWM_SCREEN) \
+  X(AMIWM_WFLAGS) \
+  X(UTF8_STRING) \
+  X(WM_CHANGE_STATE) \
+  X(WM_CLASS) \
+  X(WM_COLORMAP_WINDOWS) \
+  X(WM_DELETE_WINDOW) \
+  X(WM_PROTOCOLS) \
+  X(WM_STATE) \
+  X(WM_TAKE_FOCUS) \
+  X(_NET_SUPPORTED) \
+  X(_NET_SUPPORTING_WM_CHECK) \
+  X(_NET_WM_NAME) \
+  X(_NET_WM_STATE) \
+  X(_NET_WM_STATE_FULLSCREEN) \
+  X(__SWM_VROOT)
+
+enum {
+#define X(atom) atom,
+  ATOMS_TABLE(X)
+  NATOMS
+#undef  X
+};
+
+extern Atom ATOMS[NATOMS];
+
 #endif