Multi-monitor - Launch Bar is lost on "Mirror Mode" after connect the HDMI/VGA monitor to restart or switch displays mode.

Multi-monitor - Launch Bar is lost on "Mirror Mode" after connect the HDMI/VGA monitor to restart or switch displays mode.

Steps to reproduce:
1. Connect to HDMI/VGA monitor,
2. switch display mode
3. Find launch bar is lost on "Mirror mode".

Expect:
There's launch bar exists in monitors.

Result:
There's no launch bar exists in internal monitor and external monitor.

Note:
due to:
https://bugs.launchpad.net/ubuntu/+source/gnome-settings-daemon/+bug/950160
there's a switch_monitor script to help switch the mode

#!/bin/bash

# call gnome-settings-dameon xrandr plugin for handing video mode switch
# the only parameter is timestamp, I use 0 here because g-s-d ignore it anyway.

dbus-send --session --print-reply \
  --dest=org.gnome.SettingsDaemon \
  /org/gnome/SettingsDaemon/XRANDR \
  org.gnome.SettingsDaemon.XRANDR_2.VideoModeSwitch int64:0

</endscript>

Surefire reproducer:

1. Connect to external display no matter HDMI or VGA
2. Open Displays in System menu, enable extended desktop mode
3. In the "Launcher placement", make the launcher stay in "right side display" only and click "Apply", notice launcher display on right side monitor only.
4. Enable "Mirror Display" in Display manger and apply.
5. Launcher is missing now.