[Forge] Option to switch between mapping types

[DarkEyeDragon]

Forge recently added support for mojang mappings. Having an option to switch between stable, snapshot or offical (mojang mappings) in the forge settings would be a great feature imo.

Which would essentially replace "MCP Version" with 2 dropdowns. The first one being the type of mappings. The second then based on the first mapping type to select the mappings version.