Language commit for frontend/backend distiction
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This is my proposal on implementing the ability to switch frontend and backend templates independently.

- This introduces a new option "template_backend" that will be set to indicate the backend template, defaults to 2k11.
- The "Engine: xxx" line in info.txt still works, but only for the frontend
- The frontend fallback chain defaults to the old "default" template to ensure that themes will work that have "old-style" HTML output
- The backend fallback chain only falls back to 2k11 and then "default"
- In the future, we will remove templates/default/admin once the mechanism is proven stable

To test this in all cases you can:

- Copy 2k11/ to 2k11-custom, edit info.txt, give it a distinct name. Edit the admin/index.tpl file to add some code to ensure that you will see that template in the backend if you pick it, or adjust the style.css or whatever.
- Copy idea/ to idea-custom, edit info.txt, add a "Engine: 2k11" line. Now you can test how a template would look like that fallsback on 2k11 instead of "default"

Those permutations can be checked and come to my mind:

- Backend: 2k11, Frontend: 2k11
- Backend: 2k11-custom, Frontend: 2k11
- Backend: 2k11, Frontend: 2k11-custom

- Backend: 2k11, Frontend: idea
- Backend: 2k11, Frontend: idea-custom

- Backend: 2k11-custom, Frontend: idea
- Backend: 2k11-custom, Frontend: idea-custom

They seem to work.

Currently the display of backend and frontend theme in templates.inc.tpl takes up some larger space. Maybe it could be prettified somehow, maybe put frontend and backend template next to each other, not beneath each other? Maybe @yellowled has some suggestions.

@onli and @ophian - please have a look at this, since you both also worked on the fallback chains. Please tell me if you have issues with this. We can easily revert, if you see this approach as not workable. This is more a "proof of concept" draft.
Merge branch '2.0' of github.com:s9y/Serendipity into 2.0