gst-launch: refer to element, pad, or object in some message strings

gst-launch: refer to element, pad, or object in some message strings

Revisit these strings now that the change regarding the message source
object in gst_element_found_tags_for_pad() got reverted. Try to refer
explicitly to what kind of element it is (element, pad, etc.) in some
cases, which is nicer than having to deduce this info (and we can
re-use the already existing translated strings for the most common
case). It also makes for better example code, since it's clear now
that the message source object doesn't have to be an element.