Update rc/client.kak to fit the tmux changes

Trim the script to remove any reference to tmux, and reflect the changes
made in the previous commit.

rc/client.kak

@@ -1,9 +1,6 @@
 # termcmd should be set such as the next argument is the whole
 # command line to execute
 decl str termcmd %sh{
-    if [ -n "$TMUX" ]; then
-        echo "'tmux split-window -h'"
-    else
     for termcmd in 'termite        -e      ' \
                    'urxvt          -e sh -c' \
                    'rxvt           -e sh -c' \
@@ -18,7 +15,6 @@ decl str termcmd %sh{
             exit
         fi
     done
-    fi
 }
 
 def -docstring 'create a new kak client for current session' \
@@ -35,16 +31,14 @@ def -docstring 'create a new kak client for current session' \
 
 def -docstring 'focus given client' \
     -shell-params -client-completion \
-    focus %{ %sh{
+    focus-default %{ %sh{
     if [ $# -gt 1 ]; then
         echo "echo -color Error 'too many arguments, use focus [client]'"
     elif [ $# -eq 1 ]; then
         echo "eval -client '$1' focus"
-    else
-        if [ -n "$kak_client_env_TMUX" ]; then
-            TMUX="$kak_client_env_TMUX" tmux select-pane -t "$kak_client_env_TMUX_PANE" > /dev/null
     else
         xdotool windowactivate $kak_client_env_WINDOWID > /dev/null
     fi
-    fi
 } }
+## Create an alias to the default focus callback to allow overriding from custom scripts (i.e. tmux)
+alias global focus focus-default