Add split-field interlacing to h265parse & interlace elements

Submitted by Zeeshan Ali @zeenix

Link to original bug (#797063)

Description

These patches add split-field (alternate) interlacing support to h265parse and interlace elements.

These patches are not yet ready to be merged as the caps lack the needed feature, which will be added soon. The point of submitting them already was to already get opinions/suggestions on the main changes.

Also I'm not entirely sure if the (short-cut) approach I took for interlace is what we would want. Suggestions welcome.