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>

doc/api/plugin-metadata.qdoc

@@ -81,6 +81,13 @@
                 If set, the respective plugin is not loaded by default but must be explicitly
                 enabled by the user. This should be done for plugins which are not expected
                 to be used by so many people as to justify the additional resource consumption.
+        \row
+            \li HiddenByDefault
+            \li Boolean
+            \li Optional. Defaults to \c false.
+                If set, the plugin is not shown in the default view in the
+                \uicontrol {About Plugins...} dialog, but only if the user requests seeing all
+                plugins. It is still shown with the \c{-version} command line option.
         \row
             \li Required
             \li Boolean