So far the backend libraries' include paths were relative within this
repository. This means the header files could not be used at their
installed location.
This change replaces them with "<>" includes to fix that problem and adds
a new include directory so building everything at once still works.
With this change it should be easier to actually split some parts into
another repository if this one would become too big.
This concerned only the built-in web view using Qt WebEngine. This change
has only effect when using Qt >= 5.14 because the API did not expose the
certificate chain before.
Martchus