Allow use of Galaxy without importing GOG library

[fm-117]

Problem
I'm currently looking into creating a "fake" integration plugin that would only import games from local csv and using a Platform ID from local config file. Why do that ? Because I'm tired of broken plugins that cease to work and take away a part of my library with them... Also I really like Galaxy UI and would like to continue using it strictly as a game library manager, importing only games I choose to.

That also means having the choice to use Galaxy with or without importing games from my GOG library.

Solution
At login screen (for GOG account), add a checkbox to ask user whether he/she wants to import games from his/her GOG account. In Settings, add a button to either Clear imported games or Import depending on the current state in the client. This part is optional as the user can always sign out and sign back in, checking or unchecking the checkbox at login screen.
This way the user is still logged in and identified but he/she may choose to use a 'blank' library as a start.

Alternatives
Manually import games (their platform will be Other), and use tags to specify the original Platform they belong to. This is not ideal as the platform tags will be mixed with other tags instead of having a true categorization by platform.