Air transport labels and symbols in violet

This improves general readability, especially at z=10-12 by changing color for airport symbols and captions to a violet tone (in line with the general use of violet for airport infrastructure) and thereby reducing the number of different label types in a blue tone and avoiding confusion, esp. with water and glacier labels - see also #1031.

Helipads (shown at z>=16) are included as per suggestion by @mkoniecz.

It also introduces new svg symbols for those two - the previous airport symbol was essentially semi-transparent leading to a very mushy appearance in case of busy backgrounds.

Rendering examples:

![airports](http://www.imagico.de/files/airport-color1.png) ![helipads](http://www.imagico.de/files/airport-color2.png)