Let packages specify help-echo of tabulated-list header

[Jonas Bernoulli]

Hello,

When using narrow columns in a tabulated-list buffer it would sometimes
be nice if the package could specify the help-echo shown when moving the
mouse over a column label.  Or can you correctly guesses what the "B>U"
"U>B" columns in Magit's repository list buffer are about?

This would be easy to do, just teach `tabulated-list-init-header' to use
the `:help-echo' property, if any, instead of a hard-coded string.  Like
it does for `:pad-right' and `:right-align'.

  Cheers,
  Jonas