Argument for 'kill' for killing a specific window (now default) or the whole client (+test)

Use 'kill window' to kill a specific window (for example only one specific
popup), use 'kill client' to kill the whole application (or X11 connection to
be specific).

src/workspace.c

@@ -244,7 +244,7 @@ void workspace_show(const char *num) {
         /* check if this workspace is currently visible */
         if (!workspace_is_visible(old)) {
             LOG("Closing old workspace (%p / %s), it is empty\n", old, old->name);
-            tree_close(old, false, false);
+            tree_close(old, DONT_KILL_WINDOW, false);
             ipc_send_event("workspace", I3_IPC_EVENT_WORKSPACE, "{\"change\":\"empty\"}");
             changed_num_workspaces = true;
         }