Disallow opening Serial monitor during compilation

Commit 6d5597b introduced a guard against multiple concurrent operations.
This guard also avoid any real serial monitor "open" during the compile+upload phase, but it didn't handle keyboard shortcuts.

Fixes arduino#6015

Author: facchinm

app/src/processing/app/Editor.java

@@ -2168,6 +2168,8 @@ synchronized public void handleExport(final boolean usingProgrammer) {
     console.clear();
     status.progress(tr("Uploading to I/O Board..."));
 
+    avoidMultipleOperations = true;
+
     new Thread(timeoutUploadHandler).start();
     new Thread(usingProgrammer ? exportAppHandler : exportHandler).start();
   }

app/src/processing/app/EditorToolbar.java

@@ -376,7 +376,6 @@ public void mousePressed(MouseEvent e) {
         // launch a timeout timer which can reenable to upload button functionality an
         if (!editor.avoidMultipleOperations) {
           editor.handleExport(e.isShiftDown());
-          editor.avoidMultipleOperations = true;
         }
         break;