[QOL] Add ability to use Mouse4 & Mouse5 buttons on the steam workshop downloader menu

[nooshstuff]

This is something that many web browsers do already, and I didn't realize I was used to using it all the until I tried using it here. Would be a nice QOL feature for browsing.

Mouse4 is usually bound to "Back"/"Previous" and Mouse5 to "Forward"/"Next"