FIX: support key event in ComboBox

Change-Id: Iac5f114ff45123f0a6b1bc24ef289a6112475183

src/slic3r/GUI/Widgets/DropDown.cpp

@@ -71,7 +71,9 @@ void DropDown::SetSelection(int n)
     assert(n < (int) texts.size());
     if (n >= (int) texts.size())
         n = -1;
+    if (selection == n) return;
     selection = n;
+    paintNow();
 }
 
 wxString DropDown::GetValue() const