Make optionally space visible (like in MS Word)

There should be a short-cut and an menu icon to toggle the visibility of normal spaces.

If visible, they should look like when they are visible in MS Word. This can be achieved by toggling the used font via CSS and create a custom font, that shows that character instead of the space, when spaces should be shown.

For line breaks we already have whitespace tags (that we also need because of how a browser deals with whitespace), but we should always enter a new line in the open segment after the new line tag, so that it really looks like a new line also for the user (we need to make sure, that no extra space is saved for this in the segment table, when saving).

Both should be true for the open segment and for closed segments as well.