fix: show locally installed games in drop-down when offline

[skaldarnar]

Contains

Problem: The launcher knows about locally installed games and could work without internet connection, but the dropdown boxes for games to start uses releases as backing information.

Solution: Take the locally installed games into account when assembling the list of releases shown in the dropdown box.

Closes #198, #199.

How to test

1. Start the launcher while being connected to the internet, and download a game.
2. Stop the launcher.
3. Disconnect from the internet.
4. Start the launcher.
5. Ensure that any locally installed games can be selected in the drop-down and started.

There might be a bunch of other things that might be affected by this change:

• what happens if there is no game installed at all (a) when offline, (b) when online
• what happens if the installed games change (e.g., by downloading or deleting an game)

Outstanding before merging

• 463bbd1 There are currently duplicate entries for the same release for installed games, as the the GameRelease fetched from online sources and the dummy GameRelease for locally installed games are not merged.
• 8cc09c9 When offline, the drop-down is initially empty, even if there are locally installed games.

[jdrueckert]

Tests out fine. On current master, the drop-down stays empty, on this branch, it's filled with the version(s) I have installed. If I delete my locally installed versions it's empty again.