Make the skin use the sidebar appearance from the Windows 10 AU

[sylveon]

They redesigned the sidebar's appearance on the AU. Updating the skin so it follows the new style seems like a good idea.

[Edgarware]

The current implementation of the sidebar (which is already pretty hacky to begin with) prevents me from dynamically changing the icons. So I wouldn't be able to change the icon color on selection, which would probably not look super great.

I'd like to, and I'll look into it, but unless I can somehow get Steam to use alpha masks it probably won't be easy.

[Edgarware]

I tried this! And got really close, but the only method of doing alpha channels I could think of conflicts with hover state colors.

But with Fluent Design coming soon and being completely impossible in the current Steam GUI framework, we might just have to live with not being up to date with our design.