* fix 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
Merge pull request #47 from alexey-lysiuk/assert_fixes

Fixes for wxWidgets asserts
Merge branch 'master' of https://github.com/sirjuddington/SLADE into osx