Fix locking scheme for double-clicks

In some cases, the events may be driven in the same thread, which can
lead to a deadlock.  Instead of waiting for the lock, simply refuse to
run if a loading system in in progress.

Fixes https://gitlab.com/kicad/code/kicad/issues/6606

(cherry picked from commit d7a6a2cc71)

kicad/tools/kicad_manager_control.cpp

@@ -597,7 +597,10 @@ int KICAD_MANAGER_CONTROL::ShowPlayer( const TOOL_EVENT& aEvent )
     KIWAY_PLAYER* player;
 
     // Prevent multiple KIWAY_PLAYER loading at one time
-    const std::lock_guard<std::mutex> lock( m_loading );
+    if( !m_loading.try_lock() )
+        return -1;
+
+    const std::lock_guard<std::mutex> lock( m_loading, std::adopt_lock );
 
     try
     {