D7 ThemeAPI is incomplete

Issue Summary

I created this issue to focus on some small changes to ThemeAPI that will make
D7 theming much more flexible and consistent. This issue is related to #812796
and #364470. I know I'm late. But, as for D7 user I think that without small
improvement, D7 will look like incomplete.

My goal is to create D7 theme using only css and template.php files. For D6
themes I always changed following things:

- submitted message (both for nodes and comments)
- comment title (with number of comment included)

In current D7 ThemeAPI I cannot change this variables without using tpl files.
Comment title is hardcoded, submitted message is hardcoded. I think, creating
translation for submitted message to change how it looks, is not the way it
should be used. I cannot achieve my no-tpl-files goal with hardcoded variables.

What I propose is to:
- make comment title as variable (like I did in patch in #812796, for
comment-wrapper.tpl.php) - now I have to use comment-wrapper.tpl.php to add,
for example, number of comments
- do not hardcode submitted message (if not an old $submitted variable (see
#364470), then, maybe something like theme('links'), which is also quite
difficult for new Drupal themers, maybe something like a theme setting).

Any other variable can be adjusted in template.php, thus I think leaving those
I mentioned will make D7 ThemeAPI incomplete.