Rewrite of tmux mouse support which was a mess. Instead of having

Rewrite of tmux mouse support which was a mess. Instead of having
options for "mouse-this" and "mouse-that", mouse events may be bound as
keys and there is one option "mouse" that turns on mouse support
entirely (set -g mouse on).

See the new MOUSE SUPPORT section of the man page for description of the
key names and new flags (-t= to specify the pane or window under mouse
as a target, and send-keys -M to pass through a mouse event).

The default builtin bindings for the mouse are:

    bind -n   MouseDown1Pane select-pane -t=; send-keys -M
    bind -n MouseDown1Status select-window -t=
    bind -n   MouseDrag1Pane copy-mode -M
    bind -n MouseDrag1Border resize-pane -M

To get the effect of turning mode-mouse off, do:

    unbind -n MouseDrag1Pane
    unbind -temacs-copy MouseDrag1Pane

The old mouse options are now gone, set-option -q may be used to
suppress warnings if mixing configuration files.