icon on main toolbar

[paulvi]

like in TCF terminals

[culmat]

I'm don't want to impose clutter on occasional users by forcing a new icon on the tool bar.
Will pick this up when implementing a preference page, see #48.
Work around with
Ctrl + Alt + Shift + E
or
Ctrl + 3

( Have to check if Ctrl + Alt + W is still working as intended on all platforms.)