non-functional theming changes.

Patch to the theming of privatemsg:

1. Better namespacing for the css classes. Not complete though, left-column etc are still there.
2. Get rid of admin option to use alternative tpl file. This should IMO be done via theme overrides - if a person wants a different look, put a modified version in the theme folder. This also allows us to focus on having one look in the main module.
3. Not combined the left two columns. This is a ToDo as IMO the current layout wastes too much space.

#3 makes this CNW.