Define CallAfter and Yield in BasicUI...

...so direct uses of wx/app.h may later be removed.  That header is one of the
files that is in wxBase, but is not acceptable in "toolkit neutral" code,
which should not directly use any version of an event loop.

libraries/lib-basic-ui/BasicUI.cpp

@@ -9,6 +9,9 @@ Paul Licameli
 **********************************************************************/
 #include "BasicUI.h"
 
+#include <mutex>
+#include <vector>
+
 namespace  BasicUI {
 Services::~Services() = default;
 
@@ -22,4 +25,40 @@ Services *Install(Services *pInstance)
    theInstance = pInstance;
    return result;
 }
+
+static std::recursive_mutex sActionsMutex;
+static std::vector<Action> sActions;
+
+void CallAfter(Action action)
+{
+   if (auto p = Get())
+      p->DoCallAfter(action);
+   else {
+      // No services yet -- but don't lose the action.  Put it in a queue
+      auto guard = std::lock_guard{ sActionsMutex };
+      sActions.emplace_back(std::move(action));
+   }
+}
+
+void Yield()
+{
+   do {
+      // Dispatch anything in the queue, added while there were no Services
+      {
+         auto guard = std::lock_guard{ sActionsMutex };
+         std::vector<Action> actions;
+         actions.swap(sActions);
+         for (auto &action : actions)
+            action();
+      }
+
+      // Dispatch according to Services, if present
+      if (auto p = Get())
+         p->DoYield();
+   }
+   // Re-test for more actions that might have been enqueued by actions just
+   // dispatched
+   while ( !sActions.empty() );
+}
+
 }