Make retrieving assets through Steam and GOG easier

This is mostly an issue for users on Mac and Linux. The amount of questions we get about this illustrates that this too difficult for many users currently.

Since the actual install mostly involves running a few commands, it would be nice if OpenRCT2 could run these for the user.

When OpenRCT2 starts up and doesn't have any assets yet, it could, for example work like this:

OpenRCT2 needs files from the original RollerCoaster Tycoon 2 in order to work.

Select which option applies to yet:
1. I already have RCT2 installed on my computer.
2. I don't have RCT2 installed, but I own it on Steam.
3. I don't have RCT2 installed, but I own it on GOG.
4. I don't own RCT2 yet.

Option 1 will lead to the current dialog box where the user selects their directory.
Option 4 will display information on how and where to get RCT2.

Option 2 could do two things:

• only use Steam itself, starting it up with the Console tab and giving users instructions (easy to implement)
• use a script to actually download RCT2 for the user (more difficult to implement, but much more user friendly).

Option 3 would work similar to option 2:

• the simplest option would be to ask the user to download the full install and tell OpenRCT2 where it is, then use innoextract to extract it.
• the more user-friendly option would be to use lgogdownloader to download the game for the user.

The issue names no source files or tests; start by locating the current dialog box for selecting the RCT2 directory. Review the Steam Console, innoextract, and lgogdownloader approaches described in the issue. Done would require an agreed implementation for retrieving or extracting assets and a tested startup flow for users without assets.