Fix assert in BrowserWindow class

Opening of Things Browser no longer triggers this assert within wxWidgets:
src/generic/treelist.cpp(1128): assert "col < GetColumnCount()" failed in DeleteColumn(): Invalid column index
Merge pull request #149 from alexey-lysiuk/fix_assert

Fix assert in BrowserWindow class
Merge branch 'master' of https://github.com/sirjuddington/SLADE into osx