Rather than fiddling around with the activation modes, this seems like a
much cleaner way to make things work.
Closes https://github.com/ppy/osu/issues/18277.
Add dummy channel `DummySelectorChannel` which should be set as the
current channel in the channel manager when the selector in the chat
overlay should be shown.
Refactors the `ChannelListItem` to not show mention pill and close
button when the channel is the dummy selector channel.
Ensure that the `ChannelList` selects the dummy channel on clicking the
selector item.
Removes `ChannelListSelector` as it is no longer needed.
Removes the `setCurrent` parameter from `ChannelManager.JoinChannel`
method as it is no longer needed.
The previous transition was supposed to be a center-anchored elastic
scale-in, but this didn't work as intended - because the multiplier
ended up inside of an auto-sized right-aligned container, the animation
itself would end up being anchored right.
Attempts to remove the scale transition resulted in a rather
jarring-looking result, so swap out the elastic scale-in for a sweep-in
effect from the top, to match the header and avoid introducing too many
directions of movement.
Delay values tweaked "to taste" - can be adjusted further if there is an
alternative set of values that feels better.