Freemarker templates render doubles in system locale

Freemarker templates render double values in the system locale instead of the locale of the current mount/channel.

Steps to reproduce:
1. use a Dutch OS, or set the locale of your OS to Dutch
2. add a field of type 'Double' to a document type (e.g. the 'news' document from Essentials' 'news' feature)
3. set a double value "0.5" in a document of that type (zero dot five)
4. edit the template that renders that document, and make it render the double too. For example, add:

${document.myDouble}

5. request the page in the site and check the rendered double value

Expected: the double renders as 0.5 (zero dot five)
Actual: the double renders as 0,5 (zero comma five)

This is especially annoying if the double is used in a fmt tag, for example:

<@fmt.formatNumber value="${document.myDouble}" type="currency"/>

Since 0,5 is not a valid Java double, rendering the template will generate an error.

It is questionable whether doubles should be rendered locale-specific at all. Probably they should always be rendered in dot-notation (zero dot five) so they remain valid Java doubles.

This mainly affects developers that are using a non-dutch OS. Servers will probably always run an OS with English locale.