Add about:config option to allow adding custom search engines

[xandaaah]

In the 'Search Shortcuts' section of the settings, there is no add button, but adding the config option 'browser.urlbar.update2.engineAliasRefresh' and setting it to true allows you to add custom search engines (it adds an add button that allows you to add search engines). It should be easy to set this to true as a default.