Keep sections when they become empty

[smortex]

When the last setting of a section was removed, the whole section was removed unless it contained white space of comments. In #532, this was changed to also remove sections that only contained space (blank lines), but it caused regressions and was reverted in #535.

For consistency, we completely suppress the auto-removal of "empty" sections: removing the last setting of a section will not remove this section anymore, just like what happens for sections with only blank lines and comments.

Also include:

• #535