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.

key-string.c

@@ -82,6 +82,19 @@ const struct {
 	{ "KPEnter",	KEYC_KP_ENTER },
 	{ "KP0",	KEYC_KP_ZERO },
 	{ "KP.",	KEYC_KP_PERIOD },
+
+	/* Mouse keys. */
+	KEYC_MOUSE_STRING(MOUSEDOWN1, MouseDown1),
+	KEYC_MOUSE_STRING(MOUSEDOWN2, MouseDown2),
+	KEYC_MOUSE_STRING(MOUSEDOWN3, MouseDown3),
+	KEYC_MOUSE_STRING(MOUSEUP1, MouseUp1),
+	KEYC_MOUSE_STRING(MOUSEUP2, MouseUp2),
+	KEYC_MOUSE_STRING(MOUSEUP3, MouseUp3),
+	KEYC_MOUSE_STRING(MOUSEDRAG1, MouseDrag1),
+	KEYC_MOUSE_STRING(MOUSEDRAG2, MouseDrag2),
+	KEYC_MOUSE_STRING(MOUSEDRAG3, MouseDrag3),
+	KEYC_MOUSE_STRING(WHEELUP, WheelUp),
+	KEYC_MOUSE_STRING(WHEELDOWN, WheelDown),
 };
 
 /* Find key string in table. */
@@ -192,7 +205,9 @@ key_string_lookup_key(int key)
 
 	/* Handle no key. */
 	if (key == KEYC_NONE)
-		return ("none");
+		return ("<NONE>");
+	if (key == KEYC_MOUSE)
+		return ("<MOUSE>");
 
 	/*
 	 * Special case: display C-@ as C-Space. Could do this below in