[UX] Show 'import game' hint/instructions in 'add game' dialog

[arielj]

This PR adds a small InfoBox at the top of the Add Game dialog to tell the users that if they are trying to add an Epic/GOG/Amazon game they own from a store, they should actually use the Import Game feature.

it's small so it's not too invasive for the people that understand the difference, but hopefully it gets the attention of new users and prevents them from using Add Game when they actually want to import.

Closes #3902

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)