Allow to re-use template definitions / global master template

It should be possible to re-use template definitions (such as document sections, text styles etc.) across multiple templates without having to manually re-create those elements per template.

This would be very useful for users who need to keep same company branding/styling guidelines for their multiple templates and can simply use a master template with the required template definitions already set as base which further changes can then be applied to.

Currently, the only workaround to this use-case is to copy an existing established template each time and then make the necessary changes.