Fixes for wxWidgets asserts

TarArchive::isTarArchive() and SPDCDataFormat::isThisFormat() no longer cause
src/common/string.cpp(1187): assert "c < 0x80" failed in FromAscii(): Non-ASCII value passed to FromAscii().

MainWindow::~MainWindow() and MapEditorWindow::~MapEditorWindow() no longer cause
src/common/wincmn.cpp(468): assert "GetEventHandler() == this" failed in ~wxWindowBase(): any pushed event handlers must have been removed