Fix inconsistencies in default extension passed to SelectFile...

... It must be empty, or else one extension not including the leading dot;
then correct the string passed to FileSelector in lib-src, which should be
a filename pattern

src/LabelDialog.cpp

@@ -631,7 +631,7 @@ void LabelDialog::OnImport(wxCommandEvent & WXUNUSED(event))
                     XO("Select a text file containing labels"),
                     wxEmptyString,     // Path
                     wxT(""),       // Name
-                    wxT(".txt"),   // Extension
+                    wxT("txt"),   // Extension
                     _("Text files (*.txt)|*.txt|All files|*"),
                     wxRESIZE_BORDER, // Flags
                     this);    // Parent