Show "No Available Printers" Instead of an Empty Printer Dropdown

A ComboBox with no items opened a one-row-high popup that DropDown never
paints, leaving a blank strip with stale pixels under the box. Don't open
the list when it is empty, and label the printer box in the print and
send-to-printer dialogs when no printer is available.

src/slic3r/GUI/SelectMachine.cpp

@@ -4087,7 +4087,7 @@ void SelectMachineDialog::update_user_printer()
     {
         m_printer_last_select = "";
         update_select_layout(nullptr);
-        m_printer_box->GetPrinterComboBox()->SetTextLabel("");
+        m_printer_box->GetPrinterComboBox()->SetValue(_L("No available printers"));
     }
 
     BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << "for send task, current printer id =  " << m_printer_last_select << std::endl;

src/slic3r/GUI/SendToPrinter.cpp

@@ -1229,7 +1229,7 @@ void SendToPrinterDialog::update_user_printer()
     }
     else {
         m_printer_last_select = "";
-        m_comboBox_printer->SetTextLabel("");
+        m_comboBox_printer->SetValue(_L("No available printers"));
     }
 
     BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << "for send task, current printer id =  " << m_printer_last_select << std::endl;

src/slic3r/GUI/Widgets/ComboBox.cpp

@@ -287,7 +287,7 @@ void ComboBox::DoDeleteOneItem(unsigned int pos)
 
 void ComboBox::ForceDropdownOpen()
 {
-    if (!IsEnabled())
+    if (!IsEnabled() || items.empty())
         return;
 
     if (!drop_down) {
@@ -398,7 +398,7 @@ void ComboBox::mouseDown(wxMouseEvent &event)
     SetFocus();
     if (drop_down) {
         drop.Hide();
-    } else if (drop.HasDismissLongTime()) {
+    } else if (!items.empty() && drop.HasDismissLongTime()) { // DropDown paints nothing for an empty list
         drop.autoPosition();
         drop_down = true;
         drop.Popup(&drop);
@@ -426,7 +426,7 @@ void ComboBox::keyDown(wxKeyEvent& event)
         case WXK_SPACE:
             if (drop_down) {
                 drop.DismissAndNotify();
-            } else if (drop.HasDismissLongTime()) {
+            } else if (!items.empty() && drop.HasDismissLongTime()) {
                 drop.autoPosition();
                 drop_down = true;
                 drop.Popup();