The JoinLastClosedChannel code was using the joinedChannels list instead
of the closedChannels list. Fixing this bug made the Ctrl+Shift+t
shortuct work as expected.
Added a list to the ChannelManager class that tracks
which tabs I closed. Works like a stack, where it adds to the end
every time I close a tab. Then added a function that uses
this list to open the last closed channel, and added a shortcut inside of ChatOverlay,
similar to how jmeng implemented shortcuts.
Code is currently untested.