PluginManager: Remove hardcoded list of plugins that cannot be disabled

Instead add an optional attribute "required" to the plugin spec. Since
the locator plugin and find plugin are gone, and I don't see a reason to
prevent disabling the text editor plugin, only Core plugin gets the new
attribute set.

Change-Id: I7a380026ea46173cf00afff4213ca1b3a2578434
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/libs/extensionsystem/pluginview.h

@@ -79,7 +79,6 @@ private:
     QList<QTreeWidgetItem*> m_items;
     QHash<PluginSpec*, QTreeWidgetItem*> m_specToItem;
 
-    QStringList m_whitelist;
     QIcon m_okIcon;
     QIcon m_errorIcon;
     QIcon m_notLoadedIcon;