Auto send invites to friends

[wunnr]

Some games (e.g. Deep Rock Galactic, PEAK) don't have any in-game UI for sending friend invites, and instead activate the overlay with either Steam_Friends::ActivateGameOverlay("LobbyInvite"), Steam_Friends::ActivateGameOverlayInviteDialog(steamIDLobby) or Steam_Friends::ActivateGameOverlayInviteDialogConnectString(pchConnectString) to open a friend invite dialog. Given that the goldberg experimental overlay is not well-supported and even broken on many Linux games, ideally there should be a way to send out invites in these kinds of games without having the overlay enabled at all.

I've added functionality that parses a steam_settings/auto_send_invites.txt file. This behaves almost exactly like the auto_accept_invites.txt functionality currently in the emu: Having a list of SteamIDs in this file will add them to an auto-send list, and if a game calls one of the three aforementioned API functions, the emu will automatically send lobby invitations to the IDs on that list, as long as they are playing the same appID. If the auto_send_invites.txt file is empty, the emu will send out invites to all friends playing the same appID.

In addition, some games (e.g. Left 4 Dead 2) will check if the overlay is enabled by calling Steam_Utils::IsOverlayEnabled() before trying to open the invite dialogs, so I've modified IsOverlayEnabled() to return true if the auto_send_invites.txt file is present, even if the experimental overlay is disabled.

Tested on L4D2 and Deep Rock Galactic.

[universal963]

Meanwhile I've got an idea, for Steam_Utils::IsOverlayEnabled(), what about reusing Settings::disable_overlay option, since I think an independent option may be more flexible and this option is not actually used when in regular builds (overlay always disabled).