Reviewed by Eric Carlson.

        [GTK] enhanced context menu for media elements
        https://bugs.webkit.org/show_bug.cgi?id=45021

        New items in the media Element context menu:
        - play/pause
        - mute/unmute
        - controls display control
        - switch to fullscreen (for video only)
        - loop playback control
        - copy media url to clipboard
        - open in new window

        Test: media/context-menu-actions.html

        * page/ContextMenuController.cpp:
        (WebCore::ContextMenuController::contextMenuItemSelected):
        * platform/ContextMenu.cpp:
        (WebCore::ContextMenu::populate):
        (WebCore::ContextMenu::checkOrEnableIfNeeded):
        * platform/ContextMenuItem.h:
        * platform/LocalizationStrategy.h:
        * platform/LocalizedStrings.cpp:
        (WebCore::contextMenuItemTagOpenVideoInNewWindow):
        (WebCore::contextMenuItemTagOpenAudioInNewWindow):
        (WebCor...