Add support for importing simple statements into class diagrams.

Class and association statements could be imported by pasting from
the clipboard or by selecting "from file" in the "new" submenu of
a class diagram context menu.

Parsing errors occured on import are logged into the log window.

See test/import/diagrams/class/examples.txt for examples of
supported statements.

BUG:380266


FIXED-IN:2.22.80 (KDE Applications 17.07.80)

Signed-off-by: Ralf Habacker <ralf.habacker@freenet.de>