Add preference for "click menu to search".

Add a hidden preference for the "click menu to search" feature,
and also disable the feature if the standard menu item is not
hidden. The preference defaults to enabled (as per current
behaviour and is "extensions.contextsearch.clickMenuToSearch").
Change zh-CN access key to "S".
Fix character encoding issues with localisation.

Change encoding to UTF-8 without BOM.
Increase version number to 0.6.1.