Implicitly capturing "this" with the capture default "=" is deprecated
with C++20. We fix this by either explicitly passing this, or by copying
the required members manually.
While this exposes some rather expensive copies like the QList
selectedItems in OBSBasic_Preview, it doesn't introduce them ("=" copies
implicitly).
This removes the need to provide an undocumented launch argument to
enable custom RTMP service configurations to use multitrack encoding
and also provide a custom configuration.
