Mod update ease of use suggestion

[Mengmoshu]

Put a button in the Manage Mods window on the Loader Mods tab to help users update mods. I'm not suggesting anything automatic since that would require MultiMC to know more about each mod. Pretty much I was just thinking a version of the Add button where you select a new mod file like Add and MultiMC replaces or disables the appropriate mod. I don't know if MultiMC can look at something like the ModID to know which file to replace/disable, but that seems like it would be better than looking at file names. If looking at the mods is too complex it might also be reasonable to replace/disable the currently selected mod.

Hopefully this is clear enough, but I'll try to check back in case it isn't.