Using :align-to with non-spaces

[Alex]

Would it be a reasonable feature request to allow some usage of
:align-to with non-spaces (or to have a special display property for
alignment)? For instance, imagine one wants to align a string in the
header-line with the text area. Compare:


(setq header-line-format
      (concat (propertize " "
                          'display
                          '(space . (:align-to 0)))
              "test"))

(setq header-line-format
      (propertize "test"
                  'display
                  '(space . (:align-to 0))))

It's not a big deal, but it's makes things look cleaner, especially when
the string in question is also being propertized.

P.S. I noticed that the above does not take into account the space taken
by `display-line-numbers'.