Support an "experimental" attribute in plugin specs.

A flag that marks a plugin to be experimental, that should not be loaded
by default.

src/libs/extensionsystem/pluginspec_p.h

@@ -62,6 +62,7 @@ public:
     QString name;
     QString version;
     QString compatVersion;
+    bool experimental;
     QString vendor;
     QString copyright;
     QString license;