FR: Add user-configurable "extra install locations" setting

[Nightblade]

Example Use Case (Windows)

1. User wants to clean their Steam installation, but it's on a non-system drive, let's say F:\Games.
2. User adds this location to an "extra install locations" setting in BleachBit.
3. All cleaners that search in C:\Program Files and/or C:\Program Files (x86) now automatically search the "extra install locations" too.
4. "Steam" cleaner will now search C:\Program Files\Steam, C:\Program Files (x86)\Steam and F:\Games\Steam

Advantages

• Allows for non-standard software installation locations without the need to edit every cleaner file.

Notes

• Could probably be named better...

edit: more specific example

[az0]

This is related to an under-the-hood feature called multi-value variables, but this feature would allow the user to specify extra values for environment variables. For example, Steam often uses the environment variable %ProgramFiles% which could also expand to the user-supplied F:\games