Eeschema: fix overzealous save file update UI event handler.

Apparently wxFileName stats the file on Windows when using the == operator
which causes slow response when adding sheets in designs saved on remote
shares.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/5596

eeschema/sch_edit_frame.cpp

@@ -813,9 +813,9 @@ void SCH_EDIT_FRAME::OnUpdateHiddenPins( wxUpdateUIEvent& aEvent )
 
 void SCH_EDIT_FRAME::OnUpdateSave( wxUpdateUIEvent& aEvent )
 {
-    SCH_SHEET_LIST sheetList( g_RootSheet );
+    SCH_SCREENS screenList( g_RootSheet );
 
-    aEvent.Enable( sheetList.IsModified() );
+    aEvent.Enable( screenList.IsModified() );
 }
 
 

eeschema/sch_screen.cpp

@@ -1668,3 +1668,17 @@ void SCH_SCREENS::BuildClientSheetPathList()
         }
     }
 }
+
+
+bool SCH_SCREENS::IsModified() const
+{
+    for( const SCH_SCREEN* screen : m_screens )
+    {
+        if( screen->IsModify() )
+            return true;
+    }
+
+    return false;
+}
+
+

eeschema/sch_screen.h

@@ -652,6 +652,13 @@ public:
      */
     bool CanCauseCaseSensitivityIssue( const wxString& aSchematicFileName ) const;
 
+    /**
+     * Check if this screen list has any modified screens.
+     *
+     * @return true if there are any modified screens in this list.
+     */
+    bool IsModified() const;
+
 private:
     void addScreenToList( SCH_SCREEN* aScreen );
     void buildScreenList( SCH_SHEET* aSheet);