[development] Strip out theme functions into a .theme file

[Frando (Franz Heinzmann)]

Now that we have modules in their own directories, I think we could move 
forward to ease pain for designers by stripping out theme functions out 
of the .module file into a .theme file.

We already did the same for installation, and there's currently a patch 
in the queue to do the same for help files.

Why we should move theme_* functions into a <modulename>.theme file:

* ease pain for designers. Currently, they have to look through all the 
module code, that is not interesting for them at all, to find a theme 
function they want to overwrite. By having them all together in a 
<modulename>.theme file, it will save a lot of time and effort for them. 
And IMO we should take a bit more care of them, especially if it does 
not hurt at all.

* Seperation of code and design has always been a main target. So I 
think it's only logical to stick to this policy here and move the V part 
of a module into a different file than the C part (according to the MVC 
pattern)

best regards,
Frando