Added omni filters

- Added omni gas filter
- Cleaned up some code in mixer.dm and omni_base.dm

(cherry picked from commit 095ab7d97ce1ae55412add1ebe856dd4f9fe3651)
Add omni devices to dispenser

- Added omni filters and mixers to pipe dispenser
- Added construction code
- A couple more cosmetic tweaks

(cherry picked from commit 71ff9f76716f35b31da60a8721c7f1d2345fabae)