VBAP GUI convention:  top == front ^= azimuth == .5

This allows to move from stereo,mono panners to VBAP and back
and also facilitates sharing pannables of all currently
existing panners with semantically similar results.

(somewhat dirty solution, this retains PBD::spherical_to_cartesian
and maps angles pretty much everywhere else)
fix invalid width when swiching to 2in2out
re-allow panners for monitoring-section (for now)
prevent stackoverflow when pannable changes to fewer params

endless loop: e.g.  2in2out -> balance (or 1in1out)
#23 0xb7ab5c17 in ARDOUR::Pannable::value_as_string
#24 0xb2ebb206 in ARDOUR::Pannerbalance::value_as_string
#25 0xb7ab5c17 in ARDOUR::Pannable::value_as_string
#26 0xb2ebb206 in ARDOUR::Pannerbalance::value_as_string
ad infinitum