panels_common_edit_x_form: Correct and unify some strings

The description for the Identifier in panels_common_edit_argument_form is wrong. It says: "'Enter a title to identify this relationship..."

I corrected this and took the chance to unify this string for all four context types.
It now says: t('Enter a name to identify this !type on administrative screens.', array('!type' =>t('required context'))) etc.

Note that I replaced "title" by "name". I did so to reduce the ambiguity of "title" in the UI. There are so many titles around, so the user will finally expect a "title" to be shown in the output. Another way to put this would be: "Enter an identifier for this !type...". Maybe that's even better...?

Please rethink this and tell me which way we should go.