Add support for request signal on BOStrab:f signals

[besentv]

The approach is pretty terrible, so I'm open for improvements.

Example rendering in Karlsruhe around Durlacher Tor.

Also I'd like to add support for more common combinations, like (pseudo-regex):
F0 [F4]? [F1-3|F5] [A1]? [St9]?

So, most likely these:

• F0 F4 F1
• F0 F4 F2
• F0 F4 F3
• F0 F4 F5
• F0 F1 A1
• F0 F2 A1
• F0 F3 A1
• F0 F4 F1 A1
• F0 F4 F2 A1
• F0 F4 F3 A1
• F0 F4 F1 St9
• F0 F4 F2 St9
• F0 F4 F3 St9
• F0 F4 F1 A1 St9
• F0 F4 F2 A1 St9
• F0 F4 F3 A1 St9

[besentv]

Mhm, I just realized that many cities actually have the A as the very first light, for example:

• Karlsruhe
• Mannheim
• Mainz
• Kassel
• Leipzig
  However these have it at the bottom:
• Nürnberg
• Erfurt

[besentv]

I think i found a better way of implementing this, using layer instances.

[Nakaner]

I would like to ask to start rendering St 9 signals on zoom level 16, not 15 like the F signals. Enlarging the signals by 50% hides more other signals.

Please fix the syntax as well (see inline comment).

Zoom level 15:

Zoom level 16:

[Nakaner]

This closing curly brace makes the syntax invalid.

[bjoernma]

As I couldnt find St9 in BOStrab (and though it is DE-VBK:st9 in the key definition) it seems that its in most cities more of a railway:signal:main:PT_priority=requested;off in different optical representation. I wouldnt mind if the DE-BSVG version which is basically a single round lamp would be an "A" instead as its the meaning that counts for me. So it would be nice for me (?) to add the PT_priority tag as well.
And I (!) wouldnt mind as well whether its on top or at the bottom, but if its simple done, why not...