movement in markups like TeX's `em` and `ex` units

[Werner LEMBERG]

Consider the following snippet:

```
Emaj = \markup { E \raise #1 maj }

\markup \Emaj

\markup \fontsize #10 \Emaj
```

As can be seen, `\raise #1` is not the right value in the second case.
What I would like to have instead is an equivalent to TeX's relative
units `em` and `ex`, being multiples of the width of glyph 'm' and the
height of glyph 'x', respectively (HTML/CSS has that too, BTW).

How can I do that in LilyPond?  I could imagine code like

```
Emaj = \markup { E \raise #(* 1 ex) maj }
```

In other words, font-related command like `\fontsize` could set up
markup-wide entities `ex` and `em` that can be used for offsets
depending the current (scaled) font dimensions.

Or maybe

```
Emaj = \markup { E \raise #(ex 1) maj }
```

where function `ex` examines the current font and gets the 'ex' value?
Ditto for a function `em`.


    Werner