Runways are fairly large features in the landscape, often highly visible
from above. The line rendering style of normal highways is not ideal
when zooming in on aerodromes, because the line representing the runway
diminishes into something much narrower than the actual area.

This commit introduces rendering based on the width tag of
`aeroway=runway`, and uses that to set the line-width. It defaults to
18 m if this tag is missing (this is on the narrow side, so it shouldn't
cause strange rendering where `width` is missing).

To help identify these large grey areas as runways, a centreline is
drawn.

Stop rendering its `ref` (this is better left to the main body of the
`aeroway=runway` and draw the arrows that are typically used to mark a
displaced threshold in aviation.

Width rendered as with aeroway=runway.

Treat `aeroway=taxiway` the same as `aeroway=runway`, and use its width
for rendering. `area:aeroway` is rendered for values of `runway`,
`taxiway`, and `stopway`. Colours of runway, taxiway, and aeroway area
are adjusted to work well together.

Also move `aeroway=apron` to the other aeroway areas.