tmux: mouse back on

Mouse mode was turned off while hunting the vanishing status bar. It was
never the cause -- the nested window formats were starving status-right of
width -- so this restores click-to-select, drag-to-resize and wheel scroll.

.tmux.conf

@@ -69,7 +69,7 @@ bind E setw synchronize-panes off
 # set first window to index 1 (not 0) to map more to the keyboard layout...
 set-option -g base-index 1
 set-window-option -g pane-base-index 1
-set-window-option -g mouse off
+set-window-option -g mouse on
 
 # color scheme (styled as vim-powerline)
 set -g status-left-length 52