Add action to toggle ‘visualize whitespaces’ in editors

Add action, in ‘View’, that lets the user toggle the whitespace markers
in editors. Whitespace markers are the small dots/arrows etc. marking
spaces, tabs and new-lines. To some translators this can make it hard
to see the text, hence the reason for making it possible to toggle.

Change-Id: I484b51b078c84dacc1fe534b935dd9f8ce12de42
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>