wings: phase γ — rename X11-only files to *_x11.c

Mechanical rename to make architectural intent visible:

  dragsource.c       → dragsource_x11.c
  dragdestination.c  → dragdestination_x11.c
  selection.c        → selection_x11.c
  winputmethod.c     → winputmethod_x11.c
  wballoon.c         → wballoon_x11.c
  wappresource.c     → wappresource_x11.c

Each is essentially a wrapper around X11 features (XDND, X11
selection, XIM, XShape, X11 WM hints) with no Wayland equivalent
yet.  These files keep providing their public WINGs API symbols
unchanged, so build+link is identical at this point.

A future commit will add Wayland counterparts (or no-op stubs) and
wire a configure option that conditionally excludes *_x11.c when
the build is Wayland-client-only.  Doing this now would force a
choice between linkable but Wayland-broken (no stubs) or partial
double-implementation (some functions stubbed); the right answer
needs vtable dispatchers in shared *.c files for each, which is
the next round of surgery.

WINGs/Makefile.am

@@ -29,12 +29,12 @@ libWINGs_la_SOURCES = 	\
 	configuration.c \
 	dragcommon.c \
 	dragcommon_x11.c \
-	dragdestination.c \
-	dragsource.c \
+	dragdestination_x11.c \
+	dragsource_x11.c \
 	rgb.h \
-	selection.c \
-	wappresource.c \
-	wballoon.c \
+	selection_x11.c \
+	wappresource_x11.c \
+	wballoon_x11.c \
 	wbox.c \
 	wbrowser.c \
 	wbutton.c \
@@ -55,7 +55,7 @@ libWINGs_la_SOURCES = 	\
 	wfontpanel.c \
 	widgets.c \
 	widgets_x11.c \
-	winputmethod.c \
+	winputmethod_x11.c \
 	wlabel.c \
 	wlist.c \
 	wmenuitem.c \