wings: rename wcolorpanel.c → wcolorpanel_x11.c

The color picker dialog uses XGetImage / XGrabPointer / XShape /
XPutImage / XCopyArea to implement the magnify-glass screen picker.
Wayland intentionally does not allow reading pixels from other
windows (security), so the screen-pick feature has no Wayland
equivalent.  The whole file is therefore X11-only.

A future commit will provide a stub that returns a wwarning when
WMShowColorPanel is called in Wayland mode.

WINGs/Makefile.am

@@ -41,7 +41,7 @@ libWINGs_la_SOURCES = 	\
 	wcolor.c \
 	wcolor_x11.c \
 	wcolor_wl.c \
-	wcolorpanel.c \
+	wcolorpanel_x11.c \
 	wcolorwell.c \
 	wconfig.h \
 	wevent.c \