Use file absolute path when determining title bar indicators

Fixes https://gitlab.com/kicad/code/kicad/issues/8449

eeschema/sim/sim_plot_frame.cpp

@@ -322,7 +322,8 @@ void SIM_PLOT_FRAME::initWorkbook()
 
 void SIM_PLOT_FRAME::updateTitle()
 {
-    wxFileName filename = m_simulator->Settings()->GetWorkbookFilename();
+    wxFileName filename = Prj().AbsolutePath(
+            m_simulator->Settings()->GetWorkbookFilename() );
 
     bool readOnly = false;
     bool unsaved = false;