[Therion] Symbol for fixed ladder

[Benedikt Hallinger]

Hello, thank you for your elobaration on the steps.

The difference to "line:*-ladder" is, that steps are not ladders ;)
this has influences on the legend, for example, as well as that with 
"line:steps" it is not correctly tagged in the data (for example if someone 
would want to list all scraps containing ladders by greping through the 
dataset searching for "line:fixed-ladder" lines).

There is already the correct symbol defined in the source, however currently 
it is only a red line and i wanted to implement that as i have a need for it 
:)

Please see https://github.com/therion/therion/pull/36#issuecomment-275244962 
for an rendered example that shows what i mean graphically.

With bet regards,
Beni