Rewrite Speedbar expansion for all descendants (bug#73533)

Rewrite Speedbar expansion for all descendants (bug#73533)

Rewrite 'speedbar-expand-line-descendants' to avoid getting into
an infinite loop by reaching max-lisp-eval-depth.  The new
method avoids querying and displaying information for every
movement, instead using a single message to indicate that
expansion is in progress, and so is significantly faster.  The
narrowing per item introduced by the fix for bug#35014 is
removed because it prevented expanded descendant items when the
top-level item was already expanded.
* lisp/speedbar.el (speedbar--get-line-indent-level): New
function to return the indentation level of the current line.
(speedbar-expand-line-descendants): Use simpler line motion and
no recursion.  Output messages indicating when expansion is in
progress and when it is completed.  Fix expansion of descendants
where the top-level item was already expanded.

(cherry picked from commit bcc4e64fa7aab9f44e3a7bcdf651a32ec52866c2)