clear selection on text input

Entering upcase chars would auto-select them
Seems I don't need upcase chars very often ;-)

src/FbTk/TextBox.cc

@@ -405,6 +405,7 @@ void TextBox::keyPressEvent(XKeyEvent &event) {
         std::string val;
         val += keychar[0];
         insertText(val);
+        m_select_pos = std::string::npos;
     }
     if ((event.state & ShiftMask) != ShiftMask)
         m_select_pos = std::string::npos;