Extract X11 functions used by wxGTK in a separate header

This makes src/unix/displayx11.cpp almost readable as it's not littered
by "#ifndef __WXGTK20__" checks everywhere any more -- instead this file
is just not compiled as part of wxGTK2 at all any longer (it is still
included in wxGTK1 as well as wxX11 itself and wxMotif).

wxGTK code also can just include the new wx/unix/private/displayx11.h
instead of having to declare all the X11 functions it uses manually.

There should be no changes in behaviour, this is just a clean up.

build/cmake/files.cmake

@@ -1206,7 +1206,6 @@ set(XWIN_LOWLEVEL_SRC
     src/generic/caret.cpp
     src/generic/imaglist.cpp
     src/unix/dialup.cpp
-    src/unix/displayx11.cpp
     src/unix/fontenum.cpp
     src/unix/fontutil.cpp
     src/unix/uiactionx11.cpp
@@ -1513,6 +1512,7 @@ set(GTK1_LOWLEVEL_SRC
     src/gtk1/utilsgtk.cpp
     src/gtk1/win_gtk.c
     src/gtk1/window.cpp
+    src/unix/displayx11.cpp
 )
 
 set(GTK1_LOWLEVEL_HDR
@@ -1656,6 +1656,7 @@ set(MOTIF_LOWLEVEL_SRC
     src/x11/pen.cpp
     src/x11/region.cpp
     src/x11/utilsx.cpp
+    src/unix/displayx11.cpp
 )
 
 set(MOTIF_LOWLEVEL_HDR
@@ -1844,6 +1845,7 @@ set(X11_LOWLEVEL_SRC
     src/x11/utils.cpp
     src/x11/utilsx.cpp
     src/x11/window.cpp
+    src/unix/displayx11.cpp
 )
 
 set(X11_LOWLEVEL_HDR