Separate the boards menu per platform

Previously, the Tools->Boards menu was one long list, divided into
different platforms by (unselectable) headers. When more than one or two
platforms were installed, this quickly results in a very long list of
boards that is hard to navigate.

This commit changes the board menu to have a submenu for each platform,
where each submenu contains just the boards for that platform.

This fixes part of arduino#8858.

app/src/processing/app/Base.java

@@ -1472,23 +1472,18 @@ public void actionPerformed(ActionEvent actionevent) {
     JMenuItem firstBoardItem = null;
 
     // Cycle through all packages
-    boolean first = true;
     for (TargetPackage targetPackage : BaseNoGui.packages.values()) {
       // For every package cycle through all platform
       for (TargetPlatform targetPlatform : targetPackage.platforms()) {
 
-        // Add a separator from the previous platform
-        if (!first)
-          boardMenu.add(new JSeparator());
-        first = false;
-
         // Add a title for each platform
         String platformLabel = targetPlatform.getPreferences().get("name");
-        if (platformLabel != null && !targetPlatform.getBoards().isEmpty()) {
-          JMenuItem menuLabel = new JMenuItem(tr(platformLabel));
-          menuLabel.setEnabled(false);
-          boardMenu.add(menuLabel);
-        }
+        if (platformLabel == null)
+          platformLabel = targetPackage.getId() + "-" + targetPlatform.getId();
+
+        JMenu platformBoardsMenu = new JMenu(tr(platformLabel));
+        MenuScroller.setScrollerFor(platformBoardsMenu);
+        boardMenu.add(platformBoardsMenu);
 
         // Cycle through all boards of this platform
         for (TargetBoard board : targetPlatform.getBoards().values()) {
@@ -1497,7 +1492,7 @@ public void actionPerformed(ActionEvent actionevent) {
           JMenuItem item = createBoardMenusAndCustomMenus(boardsCustomMenus, menuItemsToClickAfterStartup,
                   buttonGroupsMap,
                   board, targetPlatform, targetPackage);
-          boardMenu.add(item);
+          platformBoardsMenu.add(item);
           boardsButtonGroup.add(item);
 
           if (firstBoardItem == null)