added i18n support for security PaletteFormComponent's (web-security)

See [GEOS-5573](https://jira.codehaus.org/browse/GEOS-5573)

This patch adds support for internationalization of palette headers of the web-security module. Sublcasses of PaletteFormComponent can override default headers (see **PaletteFormComponent.selectedHeader** and **PaletteFormComponent.availableHeader** in _GeoServerApplication.properties_) by overriding the two methods 
* getSelectedHeaderPropertyKey()
* getAvaliableHeaderPropertyKey()