Cleanup: unify coding style, fix typos, minor apidoc fixes

* apply the common 4 spaces indentation, remove tabs
* unify apidoc formatting
* fix lots of typos
* unify formatting of license text
* move include guards below license text
* unify coding style in general, in a way that makes krazy more happy
* use KF5 style Qt includes (i.e. <QClass> instead of <QtModule/QClass>)

Side note: the plain files become slightly bigger, but the xz compressed
tarball is a bit smaller