Fix switch-default warning

elementmodel.cpp:122: warning: C4065: switch statement
contains 'default' but no 'case' labels

Change-Id: I58316a3842ccf53f9ae3c33ca9bf62c1966caf59
Reviewed-on: https://codereview.kdab.com/10984
Reviewed-by: Continuous Integration <build@kdab.com>
Reviewed-by: Volker Krause <volker@kdab.com>
CMakeLists.txt - add compiler warning -Wreorder, where available