add German signals Bü4 and Pf1 (whistle signal)
add German whistle signs to style

z-index is set to 500 in order that a main signal tagged at the same node
has a higher rendering priority
rename Bü1 (DS variant from Deutsche Reichsbahn) to Pf1
rename from Pf1 (DS) to Bü4 (DS)
add German Bü4 signal with two stripes
add German Pf1 signal with two stripes
add German Bü4 and Pf1 with two stripes to style
Merge remote-tracking branch 'upstream/master' into pfeiftafeln-icons
Merge branch 'master' into pfeiftafeln-icons
Merge pull request #71 from Nakaner/pfeiftafeln-icons

Added german whistle posts (Pfeiftafeln)
Added german whistle post (Pfeiftafel) to signalling layer
Added german Vr semaphore signals to signalling layer legend, minor corrections
Removed deprecated signal icons
add icons for German Hl signals
Merge remote-tracking branch 'upstream/master' into hl-signale
Merge pull request #72 from Nakaner/hl-signale

Added german Hl signal icons