Add wxPropertyGridPageState::GetColumnFullWidth().

Add wxPropertyGridPageState::GetColumnFullWidth().

This method allows to get the full width needed by wxPropertyGrid and is
useful for determining the space to allocate for it, e.g. when creating a
popup showing the property grid, as shown by the changes to the sample.

Closes #15368.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74628 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775