WesnothMap: add code used by the [map] tag support.

It must be configured in the ACP with the following parameters:

bbCode spec.: [map]{TEXT}[/map]

HTML: <form name="Wesnoth Map Button" target="_blank"
            action="includes/WesnothMap.php" method="post">
            <input type="hidden" name="wesnoth_map_data" value="{TEXT}">
            <input type="submit" value="Display Map">
      </form>
(cherry picked from commit ec8681a)

(cherry picked from commit 3f20b50)

Author: irydacea