More events timings and one new event picture

After collecting feedback and some intensive testing:

Transmission: differentiated starting vectors. There are now two 'touch' and two 'shedding' with numbers adjusted accordingly. 'Touch' doesn't increase air transmission any more. Ungulate mutation rate reduced a little.

Symptoms: cost progression of pony traits adjusted to offset higher DNA cost of transmission.
Some trait dependencies fixed as well as a few tweaks on impact on mutation rate.

Abilities: several adjustments to cure and research impacts as well as adjustments to mutation rate development. Some costs adjusted.

Events: New event added. Adjusted several event timings and effects for smoother gameplay. Also changed many event trigger parameters to make them reliably happen.

Fixed typos, added/changed some event pictures, tweaked research impact of 'forehooves' mutation.

Changed some artwork

Scenario finalised.