Support for playnite 9

[sharkusmanch]

• Updated for SDK changes in Playnite 9 (SDK 6.0.0)
• A game can now have multiple platforms. For now, just use the first platform and don't backup if there is more than one platform. The current platform backup system will likely need to be reworked, but seemed out of scope for this PR.
• Added manifest file for new built-int add-on browser/installer https://github.com/JosefNemec/PlayniteAddonDatabase. Will submit a PR to add on repo when this is merged.

[darklinkpower]

Hello, sorry for interfering in the PR but I gave it a quick read and have some advice:

• For matching the pc platform, it's probably a good idea to use new SpecificationIdProperty property in platforms. It should be safer and more reliable because the Platform name can be changed and the extension will stop functioning correctly if it's changed, while the specification Id is always the same. The specification Id for PC games is pc_windows https://playnite.link/docs/devel/api/Playnite.SDK.Models.Platform.html#Playnite_SDK_Models_Platform_SpecificationId
• I noticed that you are indexing in the platforms property, but this will cause issues if platforms is null. You can use the null conditional operator ? as it was before to prevent this
• This one is up to preference and up to what @mtkennerly decides, but maybe saving the game to a game var would be simpler to prevent so many changes from game to arg.Game
• The new plugin template comes with changes to the ...Settings class but it was not changed for this PR. I'm not sure if settings still works without changing this but I still highly recommend to change since it's easier to work with

[sharkusmanch]

For matching the pc platform, it's probably a good idea to use new SpecificationIdProperty property in platforms. It should be safer and more reliable because the Platform name can be changed and the extension will stop functioning correctly if it's changed, while the specification Id is always the same. The specification Id for PC games is pc_windows https://playnite.link/docs/devel/api/Playnite.SDK.Models.Platform.html#Playnite_SDK_Models_Platform_SpecificationId

Good catch, thanks. Done

I noticed that you are indexing in the platforms property, but this will cause issues if platforms is null. You can use the null conditional operator ? as it was before to prevent this

Good catch, thanks. Done

This one is up to preference and up to what @mtkennerly decides, but maybe saving the game to a game var would be simpler to prevent so many changes from game to arg.Game

Agreed, also gives a smaller diff in this case. Done.

The new plugin template comes with changes to the ...Settings class but it was not changed for this PR. I'm not sure if settings still works without changing this but I still highly recommend to change since it's easier to work with

The settings do work. I would like to keep the scope of this PR small. Given that the settings still work, I would prefer to leave as is for now.

[mtkennerly]

Sorry for the delay in looking at this! I've been a bit busy personally, but I'm going to make time to review this tonight or tomorrow.