Support 'hidden' plugins

Plugins can be hidden in the "About Plugins" view by default. Users can
still make them all visible, but the default view can be made less noisy
by hiding plugins that only exist as a base for other plugins.
Plugins that can not run on the current platform are hidden by default
as well.

Change-Id: Iaf2f751c4ea4b3afc605bbbea6611eea042e62c7
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/libs/extensionsystem/pluginspec_p.h

@@ -71,6 +71,7 @@ public:
     QString version;
     QString compatVersion;
     bool required = false;
+    bool hiddenByDefault = false;
     bool experimental = false;
     bool enabledByDefault = true;
     QString vendor;