Fp config from socket

[zeffii]

This makes it possible to configure a FilePath Node as a function of the kind of FilePath socket with which it is linked.

• this introduces a new property (StringProperty) of the SvFilePathSocket called filepath_node_mode, the FilePath Node will look at this property use the mode therein to configure the SvFilePathFinder Operator.

SvFilePathFinder Operator can behave according to the kind of files it's used for.

i'm not sure this approach is sufficient. While this makes some functionality easier, it may interfere with flexibility in its current state. It may still be useful to add a features to draw_buttons function of the SvFilePathFinder Operator, which can let the user reset the mode - as a fail safe incase the automatic mode resulted in something undesirable.

[zeffii]

resolves #4446

if no-one has objections. this will evolve into something more useful as more filetypes are added

[zeffii]

not all nodes that use FilePath socket will make use of this feature, as most are currently self appending an extension, and only using the FilePath node to provide a location and file_name (without .ext). I'm reluctant to make changes to these nodes, even to the point where I might only commit the changes to the single FCStd write node. Or just close the PR.