[client] x11: add support for i3 global full screen toggle

This commit adds an interface to the X11 display server code to support
various window manage specific features, such as in this case, the i3
global full screen toggle.

This feature specifically uses the i3 IPC to cause looking glass to go
full screen across all monitors if the new option `i3:globalFullScreen`
is enabled.

client/displayservers/X11/CMakeLists.txt

@@ -17,6 +17,9 @@ add_library(displayserver_X11 STATIC
   x11.c
   atoms.c
   clipboard.c
+
+  wm/default.c
+  wm/i3.c
 )
 
 add_definitions(-D GLX_GLXEXT_PROTOTYPES)
@@ -28,5 +31,5 @@ target_link_libraries(displayserver_X11
 
 target_include_directories(displayserver_X11
   PRIVATE
-    src
+    .
 )