Add KSC mod tray and ability for mods to add their button to the tray

[Falki-git]

This is similar to how app buttons are added in Flight and OAB scenes.
KSC tray is indended for mods that are used outside Flight and OAB.
Upcoming HUMANS and (probably) Life Support mods will use it.

Difference in relation to Flight/OAB trays is that KSC menu doesn't operate as a toggle, but rather like a general button; meaning there is no toggled state.

Example usage:

Appbar.RegisterKSCAppButton(
      "Text that will be displayed for the button",
      ToolbarKscButtonID,
      AssetManager.GetAsset<Texture2D>($"{SpaceWarpMetadata.ModID}/images/icon.png"),
      () =>
      {
            _isWindowOpen = !_isWindowOpen;
      }
);

[cheese3660]

LGTM, will merge when I get home