GtkBackend: Fix crash in Picker init when selection is nil (#139)

* Fix crash in Picker init when selection is nil

* Add PickerExample

* Revert "Add PickerExample"

This reverts commit 55ca328.

* Fix crash in Picker init when selection is nil

Author: valeriyvan

Sources/GtkBackend/GtkBackend.swift

@@ -754,7 +754,7 @@ public final class GtkBackend: AppBackend {
         )
 
         picker.notifySelected = { picker, _ in
-            if picker.selected == GTK_INVALID_LIST_POSITION {
+            if picker.selected == Int(Int32(bitPattern: GTK_INVALID_LIST_POSITION)) {
                 onChange(nil)
             } else {
                 onChange(picker.selected)
@@ -765,7 +765,7 @@ public final class GtkBackend: AppBackend {
     public func setSelectedOption(ofPicker picker: Widget, to selectedOption: Int?) {
         let picker = picker as! DropDown
         if selectedOption != picker.selected {
-            picker.selected = selectedOption ?? Int(GTK_INVALID_LIST_POSITION)
+            picker.selected = selectedOption ?? Int(Int32(bitPattern: GTK_INVALID_LIST_POSITION))
         }
     }