Skill bar in HUD (mauris)

Enable with `show_skill_bar = true` in your rcfile.
Adds one line to the HUD, which removes one line from monster list.
This new line lists the skills currently enabled for training alongside
their respective level.

An example:
    Dodging*1.7  Stealth 3.2  Axes 0.0

The skill bar can fit up to four skills. If more than four skills are
enabled for training, the first four are displayed and an ellipsis
(like in the monster list) indicates that there is more to see.

Skills are sorted by skill level in decreasing order. Focused skills
appear before non-focused ones, and are marked with a trailing asterisk.

Skill names will be abbreviated to three or four characters each in case the
full skill names do not fit the HUD. An example of this and the ellipsis:
    Dod*10.1  Ste 24.4  Fgt 2.0  Axe 0.0   (…)

The short skill name abbreviations try to stick close to the ones the manual
uses (?%), but some have been slightly adjusted. A few of them use four-char
abbreviations. In very rare corner cases this could lead to the last skill's
decimal place being overwritten by the ellipsis. Those cases of course will
become more common if more skills were to use four chars instead of three.
I believe the current set is a fair trade-off between interface constraints
and usability.