Hatchery: Add an option to keep farming for mega requirement

[Farigh]

The mega-evolution requirements need to be reached twice:

• The 1st time to mega-evol the pokémon (resulting in loosing the required stat on the base pokémon)
• A 2nd time to allow EV farming

An advanced setting is now available to set the wanted behaviour.

Since this behaviour is only relevant in real evolution challenge, the setting will only be shown in this case.

Fixes #283