wxDataViewCtrl::GetSelection() now always returns invalid item if more than
a single item is selected in a multi-selection control.
Also add HasSelection() and GetSelectedItemsCount() to allow checking if any
items are selected.
Updated the documentation, all ports and added a test for all these functions.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68844 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
CPPUNIT_ASSERT_EQUAL() gives more informative error messages in case of
failure which is important when running tests unattended on build slaves.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68843 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
