Add checksum skip checkbox to settings window

[animeavi]

Fixes #49

[subvod]

Thanks for this. Makes patching expansions a lot less painful.

[Miepee]

@Alcaro is there anything specifically that's blocking this PR?

[Alcaro]

As mentioned in #49, I'm not a huge fan of this approach, so I don't want to merge it. But it's properly written, and it adds a feature people ask for, so I don't want to reject it just because I'm a cranky lil shit.

That doesn't leave a whole lot of options.

[Miepee]

If the point is to protect noobs from shooting themselves in the foot, the GUI label could be changed DANGEROUS: Ignore checksum when patching and/or pop up a dialog box asking the user if they really want to disable them and explain them briefly the consequences.