Instead of assuming that the default created window in a new session has
a index of `0`, we now check window index for the session with Tmux's
list-windows command.
Additionally, the default window is now moved to index `999` instead of
`99`, cause I'm paranoid and probably stupid :)
After some experimentation it turns out that if input to send-keys is
not a known key sequence like `C-m` or `C-l` for example, it it
automatically treated as literal input.
Negating the need to use it and hence check if it's supported.
