Allow custom prefix for Browser Search entries

Prior to this, the prefix for OpenSearch entries generated by
browser_search_plugin.php was hardcoded to 'MantisBT'.

This defines a new '$g_search_title' config option (which defaults to
the value of $g_window_title) to be used as prefix, allowing

• administrators to customize the name of their browser search entries
• users to have distinct search engines for multiple MantisBT instances

$g_search_title should be shorter than 8 chars to be compliant with the
maximum size of 16 chars for the ShortName element as per OpenSearch
specification [1]

Fixes 0011964

[1] http://www.opensearch.org/Specifications/OpenSearch/1.1