Simplifying mode-line customization

[Chong Yidong]

I've been looking into simplifying mode line customization.  One of the
problems is that many of the mode line elements in mode-line-format are
strings with the same explicit help-echo property "mouse-1: Select (drag
to resize)..."  This makes mode-line-format rather verbose and annoying
to view and customize.

One fix is to introduce a new variable that serves as a default
help-echo on any part of the mode line which lacks a help-echo property.
Does anyone see a problem with this?