Only rescan libraries folders when really needed

Scanning libraries is an heavy task if the sketchbook becomes huge;
This patch targets two points:

- remove the rescan() after setLibrariesFolders(), which already performs a rescan
- call setLibrariesFolders() only when the folder list has changed
  - This ensures that no scan is performed when changing board in the same architecture

Could mitigate #6350

arduino-core/src/cc/arduino/contributions/libraries/LibrariesIndexer.java

@@ -60,7 +60,7 @@ public class LibrariesIndexer {
   private LibrariesIndex index;
   private final LibraryList installedLibraries = new LibraryList();
   private final LibraryList installedLibrariesWithDuplicates = new LibraryList();
-  private List<File> librariesFolders;
+  private ArrayList<File> librariesFolders;
   private final File indexFile;
   private final File stagingFolder;
   private File sketchbookLibrariesFolder;
@@ -101,11 +101,15 @@ private void parseIndex(File file) throws IOException {
     }
   }
 
-  public void setLibrariesFolders(List<File> _librariesFolders) {
+  public void setLibrariesFolders(ArrayList<File> _librariesFolders) {
     librariesFolders = _librariesFolders;
     rescanLibraries();
   }
 
+  public ArrayList<File> getLibrariesFolders() {
+    return librariesFolders;
+  }
+
   public void rescanLibraries() {
     // Clear all installed flags
     installedLibraries.clear();

arduino-core/src/processing/app/BaseNoGui.java

@@ -87,7 +87,7 @@ public class BaseNoGui {
   public static Map<String, LibraryList> importToLibraryTable;
 
   // XXX: Remove this field
-  static private List<File> librariesFolders;
+  static private ArrayList<File> librariesFolders;
 
   static UserNotifier notifier = new BasicUserNotifier();
 
@@ -679,8 +679,9 @@ static public void onBoardOrPortChange() {
     // Libraries located in the latest folders on the list can override
     // other libraries with the same name.
     librariesIndexer.setSketchbookLibrariesFolder(getSketchbookLibrariesFolder());
-    librariesIndexer.setLibrariesFolders(librariesFolders);
-    librariesIndexer.rescanLibraries();
+    if (librariesIndexer.getLibrariesFolders() == null || !librariesIndexer.getLibrariesFolders().equals(librariesFolders)) {
+      librariesIndexer.setLibrariesFolders(librariesFolders);
+    }
 
     populateImportToLibraryTable();
   }