That's sort of it. ItemButtonTemplate is now the base widget ItemButton. It appears to have all the same elements (textures fontstrings etc.) using the same names and parent keys as the original template.
Instead of
Code:
<Button name="ContainerFrameItemButtonTemplate" inherits="ItemButtonTemplate" virtual="true"/>
you have
Code:
<ItemButton name="ContainerFrameItemButtonTemplate" mixin="ContainerFrameItemButtonMixin" virtual="true"/>
and to create the actual button you will
Code:
<ItemButton inherits="ContainerFrameItemButtonTemplate"/>
The lines with "virtual="true" means these are not real, created in-game widgets, they are just a set of instructions that can be used to make one or more real and identical widgets later on.