FvwmEvent: add monitor_focus event

The monitor_focus event fires when the focused window changes to a
different monitor from the currently active one.  Enable and document
this.

Fixes #228

modules/FvwmEvent/FvwmEvent.1.in

@@ -192,6 +192,7 @@ e.g. for audio-events:
 	*FvwmEvent: montior_enabled beep.au
 	*FvwmEvent: montior_disabled beep.au
 	*FvwmEvent: montior_changed beep.au
+	*FvwmEvent: montior_focus beep.au
 .sp
 .fi
 The window related event handlers are executed within a window
@@ -200,11 +201,10 @@ PassId is not needed.
 
 The monitor_* events do not operate in a window context (as there
 isn't one), but react to when a monitor is plugged in (enabled),
-unplugged (disabled), or changed (resized/rotated, etc., which
-will only be true if the monitor is already active).  In all cases,
-the monitor name is is passed through to the command, hence the
-following example prints out the changed monitor's name, and
-width/height values:
+unplugged (disabled), focused (focus) or changed (resized/rotated, etc., which
+will only be true if the monitor is already active).  In all cases, the
+monitor name is is passed through to the command, hence the following example
+prints out the changed monitor's name, and width/height values:
 
 .nf
 .sp

modules/FvwmEvent/FvwmEvent.c

@@ -155,6 +155,7 @@ static event_entry extended_message_event_table[] =
 	EVENT_ENTRY( "monitor_enabled", 0 | ARG_EXPECTS_CHAR),
 	EVENT_ENTRY( "monitor_disabled", 0 | ARG_EXPECTS_CHAR),
 	EVENT_ENTRY( "monitor_changed", 0 | ARG_EXPECTS_CHAR),
+	EVENT_ENTRY( "monitor_focus", 0 | ARG_EXPECTS_CHAR),
 	EVENT_ENTRY( "reply", 0), /* FvwmEvent will never receive MX_REPLY */
 	EVENT_ENTRY(NULL,0)
 };