Make some plugins hidden by default

Hide core plugin, hello world plugin, and plugins that only provide the
base for other plugins.

Change-Id: I7f4a29a265d760b2a759122be153c7f06bd40bd2
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cpptools/CppTools.json.in

@@ -2,6 +2,7 @@
     \"Name\" : \"CppTools\",
     \"Version\" : \"$$QTCREATOR_VERSION\",
     \"CompatVersion\" : \"$$QTCREATOR_COMPAT_VERSION\",
+    \"HiddenByDefault\" : true,
     \"Vendor\" : \"The Qt Company Ltd\",
     \"Copyright\" : \"(C) 2016 The Qt Company Ltd\",
     \"License\" : [ \"Commercial Usage\",