Reviewed by Martin Robinson.

        [GTK] Add APIs for plugin management
        https://bugs.webkit.org/show_bug.cgi?id=50827

        Add WebKitWebPluginDatabase and WebKitWebPlugin classes for basic
        plugin management at the client level. For now they provide
        "read-only" access to the plugin list.

        * GNUmakefile.am: add new files to the build.
        * tests/testwebplugindatabase.c: Test that the database contains
        the test plugin, which should always be loaded.
        * webkit/webkit.h: add new headers.
        * webkit/webkitdefines.h: add new defines.
        * webkit/webkitwebplugin.cpp: An new class representing a plugin object.
        * webkit/webkitwebplugin.h: ditto.
        * webkit/webkitwebplugindatabase.cpp: A new class that contains the list of active plugins.
        * webkit/webkitwebplugindatabase.h: ditto.
        * webkit/webkitwebplugindatabaseprivate.h: Private header.
        * webkit/webkitwebpluginprivate.h: Private header.
        * webkit/webkitwebview.cpp:
        (webkit_get_web_plugin_database): returns the WebKitWebPluginDatabase for this session.
        * webkit/webkitwebview.h: add method to header.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@73858 268f45cc-cd09-0410-ab3c-d52691b4dbfc