feat: Add <Open in Desktop App> button in header

[nolddor]

Solves #241

This PR introduces a new button (disabled by default) at the top, close to the "Install Steam" button set by Valve.
Consequently, a new option on SteamDB Extension preferences page has been introduced to manage this.

Preview

[nolddor]

G'day @xPaw let me know if this makes sense or need further refinement.

Cheers!

[xPaw]

One thing to note, both this and hiding the install button currently won't work on React pages. I just looked at the workshop beta and wishlist pages, their css classes are hashes, but it seems to be at least consistent, but I don't know if it changes between builds.

<a class="_2CYMhC951F4-" href="https://store.steampowered.com/about/">Install Steam</a>

[nolddor]

I was aware of that, do you have any idea about how to proceed on new React pages?
Also I started to play around to solve #266 until I noticed they was using React.js now...

I don't event know what's the right way to edit the DOM without making React.js to complain about "Hydration Errors" even setting the *.js files at manifest to "run_at": "document_idle" seems inefficient

[xPaw]

I don't know what the best approach to react pages is either.