Separate Dexterity parts from addon template

[MrTango]

We need small and testable packages.
We already separated the theming part.
To not add another dimension to the test matrix, we will remove the type question from the addon template entirely. To create a Dexterity CT we can use another template (sub_template).
The same could be used even for creating the needed parts for a theme.
So if we solve the whole sub_template story, we could have even smaller templates and ovoid duplication of structure in templates.

So maybe lets use the issue to discuss what options we have and to find a good solution for testing and user experience.

See also #199 and #130 for previous discussions.

@mauritsvanrees @tisto @tmassman

I'll test a bit with mrbob, how we can solve the problematic, to find the needed informations of the main template and the answered questions, to use it in sub_templates like plone_dexterity and plone_theme.
I'll come back with he results soon.

[MrTango]

In the 3.0-WIP branch is a working version with subtemplates for content_type and theme.