gui: Add menu item for opening shader cache directory

[capitalistspz]

Especially helpful for non-portable Linux users, who have shaderCache outside of the main Cemu directory

[JaimieVandenbergh]

Good for Mac users too, since that's in a slightly surprising location (~/Library/Caches/Cemu/)

[HimanshuBansal112]

I think that a way to open shader cache for each game could also be a good idea for this or future pull request.