Clarify Cancel link on message forms

On the forms where messages are written, along with send/preview buttons, there's a Cancel link. As for new message form, it really cancels that form(page) and takes you back to the overview screen, which is the expected behavior, and in-line with all the rest of Drupal. (Code suggests, that the same applies also to the cases where destination is set, although I haven't seen that in action yet.) All fine for this part.

But however, on the reply form shown below an existing thread, the Cancel link points to the very same page you're on currently. This have the "obviously broken" feeling going on, as the Cancel link doesn't cancel the page and take you out to something else/previous/not having the form (as the rest of Drupal does on Cancel links) - it only just reloads the same page, with the form being still there.

To be honest, I nearly rolled a patch to change that link's URL to 'messages', or remove the link entirely (as it doesn't make sense to truly 'cancel' something that doesn't really exist on it's own, being more of an attachment to thread page). This now illustrates an example, how misunderstood this bit of UI might be.

Only later I finally got the point, that the link is (if I get it right) supposed to just clear the form (by reloading a fresh one, without any preview too). This might be a nice/useful thing, but we need to fix the link text, as the current 'Cancel' is really misleading here IMO.

To be more in line with users' expectations and the rest of Drupal, I suggest the link being "Clear", as it only just clears anything already written in the form, without canceling the form away. (Looking at core as example, this is more similar to "Reset to defaults" on admin, and not to deletions cancel link.) Of course, this only applies to thread replies, the regular new message form still needs 'Cancel', which is the case there.

Attaching a patch for review.