Add translation support for Custom Post Types and taxonomies

[raikasdev]

Adds a way to use Polylang translations with taxonomies and CPTs. Not 100% sure if this is the best way to implement this, reviews appreciated. It also fixes the issue with slugs not being generated with pll_translatable (issue #186), problem also reported to Polylang to see if it's on their side.

This requires changes to air-helper: an action named "after_air_helper_init", to make sure air-helper's language wrapper is available. PR shall be a draft, until that or similar functionality is ready (add_action init running after air-helper, checking if function is available?)

Closes #178
Closes #186

[height]

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[raikasdev]

Gotta fix the phpcs styling tomorrow

[raikasdev]

No clue how to fix the "spaces before asterisk". Squash merge recommended because of the ugly commits.