window overview icon

First you need create a gala-window-overview.desktop file in /usr/share/applications
Second you need put this code into the file:

[Desktop Entry]
Name=Slingshot-Launcher
Comment=View and manage workspaces
Exec=dbus-send --type=method_call --dest=org.pantheon.gala /org/pantheon/gala org.pantheon.gala.PerformAction int32:6
StartupNotify=false
Icon=view-list-icons
Terminal=false
Type=Application
Categories=Utility
NoDisplay=true
X-Ubuntu-Gettext-Domain=FIXME

Step Three and final:
You need create a file called gala-window-overview.dockitem in the folder /home/user/.config/plank/dock1/launchers/
with this content:
[PlankItemsDockItemPreferences]
Launcher=file:///usr/share/applications/gala-window-overview.desktop