Don't show wxWidgets 3.2 Art images if generating for wxWidgets 3.1

[KeyWorksRW]

Description:

wxUiEditor supports generating code for wxWidgets 3.1. However, because it is built with wxWidgets 3.2, the list of Art images contains many of the new svg images that are not available in wxWidgets 3.1. The dialog box should be changed to let the user choose the version so that we can remove the items that are new to 3.2. We should also check to see if there are images new to 3.3 as well.

[Randalphwa]

We only marginally support wxWidgets 3.1, so I'm not seeing this as a bug. Marking this as low priority, since given the active plans for wxUiEditor, it's unlikely this will ever be fixed unless someone else wants to tackle it.

[Randalphwa]

Support for wxWidgets 3.1 had been dropped completely, so this change is no longer needed.