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 are logged into the log windows.

See test/import/diagrams/class/classes.txt for the supported statements.

BUG:380266
FIXED-IN:2.22.80 (KDE Applications 17.07.80)