Better Pushback Mod X-Plane 11/12

Fixes included:

Always normalize external HDG values before using them

I searched through all assignments to bp.{cur|last}_pos and the dataref seems to be the culprit. I know from my AutoDGS plugin that some old airports have unnormalized ramp headings that lead to crashes in libacfutils as well.

Possible fix for https://forums.x-plane.org/index.php?/files/file/89033-better-pushback-mod-x-plane-1112/&do=findComment&comment=393982

Some fixes for the "connect tug first" procedure

The value of the "connect tug first" procedure (e.g. on VATSIM) is that you are ready to go but have a chance to make last second changes in case you get a different route.
Prior to these patches a preplanned route was not presented for final review.

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(Please note that this plugin will remain open-source.)