Make it possible for installers to disable plugins by default.

Plugin manager reads Plugins/Ignored in installation global settings
now, and handles these plugins similar to as if they were experimental.

Change-Id: I56a0c5fe143e3fbd9261244bafece26d7b99c154
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@nokia.com>
Reviewed-by: Daniel Molkentin <daniel.molkentin@nokia.com>

src/libs/extensionsystem/pluginspec_p.h

@@ -67,6 +67,7 @@ public:
     QString version;
     QString compatVersion;
     bool experimental;
+    bool disabledByDefault;
     QString vendor;
     QString copyright;
     QString license;