CODE: forward-port bridge building code from standard
CHANGE: build low bridges by default, allow "trench bridges" without a
ramp or between elevated ways
Merge remote-tracking branch 'refs/remotes/jamespetts/way-improvements' into james-low-bridges

Conflicts:
	bauer/brueckenbauer.cc
FIX: merge some things
forward-port some more of the way height clearance code
oops, this wasn't meant to go on a branch
CHANGE: build high bridges by default, low bridges with Ctrl pressed
FIX: reinstate the test for high buildings that we shouldn't build
bridges over
rm duplicate code; build connecting roads properly
FIX: be consistent when building two-tile bridges (which don't make sense)
FIX: higher maximum cost for roads connecting to our newly-built bridge
minor fixes
FIX: removing a one/two-tile bridge shouldn't leave half-connected roads
fix removal of ramp-less "trench" bridges
build low bridges by default and high bridges when ctrl is pressed
rather than the other way around
FIX: extend elevated roads when building bridges from them
avoid segfault
Merge branch 'low-bridges' of https://github.com/pipcet/simutrans-experimental.git into way-improvements
FIX: Compile error