Locked travelnet compat

[oversword]

I needed to make a few changes to make the new version of the mod compatible with locked_travlenets

These changes are not specific to locked_travlenets and should help with extending the travelnet mod in general.

None of these changes should change the current behaviour, just make it easier for people to modify the behaviour.

Changes:

• Get the pos from the context data when travelnet.form_input_handler is called, this allows someone to override travelnet.on_receive_fields without needing to have access to the pos data
• Allow the height of the form to be sent in as an option to the primary formspec, this lets you add another unit to make space for the locked travelnet specific form elements
• Call travelnet.actions.end_input every time the form is closed, this lets you override travelnet.actions.end_input as a way of hooking into the formspec closing event
• Re-arrange the way we get data for the update actions so that we can always pass in as much info as possible into end_input

[oversword]

These changes allow this MR to work https://github.com/BlockySurvival/locked_travelnet/pull/5/files?diff=unified&w=1

Which brings locked travelnets up to speed with the latest travelnet behaviour

[oversword]

(P.S. I saw all my travelnets get wiped while writing this, it may just be because I was switching back and forth between versions, but don't test this anywhere with valuable data unless you make a backup of the data first)