browser_object_class: replace `label' with `hint'

browser_object_class: replace `label' with `hint'

The "label" field of browser object classes was a simple noun, naming the
type of datum to be collected.  This text was combined with the verb
"select" in minibuffer prompts, giving the user a hint about what to do.
However, the verb "select" is really only appropriate for those browser
object classes which use the hinting module to perform their interaction.

Therefore, "label" has been replaced by a field called "hint", which should
now be a string of both a verb and a noun, tipping the user about what
kind of data is being collected, and how.

Some commands, like "find-url" now have longer prompts than they did
before.  (Its prompt now reads "Find url in current buffer (enter URL):")
Other commands now have longer prompts, whereas before they had ambiguous
ones.  For example, the prompt of "shell-command-on-url" used to be
"Shell command:" but now it is "Shell command (enter URL):".

Browser object classes which had a label for no reason (because they do
not use a minibuffer prompt) have had their labels removed altogether.