GOG: Add ability to use folder to locate offline games installers instead of downloading using Galaxy

[tranquilcarbon]

Describe the solution you'd like
Currently, The only way to install games using the GOG plugin is to download a fresh copy from GOG Galaxy. I would like to propose the ability to add an new option to allow for users to use their offline installer folder, which contains the offline installers. This greatly speeds up large installs, since the installer doesn't need to download.