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/pluginspec_p.h

@@ -65,6 +65,7 @@ public:
     QString name;
     QString version;
     QString compatVersion;
+    bool required;
     bool experimental;
     bool disabledByDefault;
     QString vendor;