Read https://stackoverflow.com/a/23749273/11956287 for a full
explanation, but basically this is important for headers like the
interfaces/ and kdocumentviewer.h that are supposed to be used by third
partyies where it may happen that wrong things happen because the whole
class is inline in a header

For the classes that are defined in the .cpp or are not installed
headers this would not be necessary but it's not so hard to add a few
default defined destructors, so do that :)