In #112326 the socket visibility functions were updated to take the
open/closed state of panels into account for visibility of the socket
icon. However, in "hidden" (collapsed) nodes the panels should be
ignored entirely, drawing all sockets on the root level. This requires
looking at the node flags to determine socket icon visibility, so a
simple method of `bNodeSocket` is not sufficient.
This patch moves the more complex visibility queries for sockets into
`bNode`, where both node and socket flags can be accessed. These should
be used for actual visibility rather than the plain flag accessors on
`bNodeSocket`.
Lukas Tönne