LibGUI: Allow the InputBox to be of NonemptyText type

It seems like a lot (most?) places where InputBoxes are used check if
the retrieved string isn't empty anyway - make this be reflected in
the user interface, by disabling (graying out) the "OK" button when
nothing is entered, so empty input isn't a viable option at all.

Userland/Libraries/LibGUI/InputBox.cpp

@@ -71,6 +71,7 @@ void InputBox::build(InputType input_type)
 
     switch (input_type) {
     case InputType::Text:
+    case InputType::NonemptyText:
         m_text_editor = label_editor_container.add<TextBox>();
         break;
     case InputType::Password:
@@ -113,6 +114,13 @@ void InputBox::build(InputType input_type)
     };
     m_text_editor->set_focus(true);
 
+    if (input_type == InputType::NonemptyText) {
+        m_text_editor->on_change = [this] {
+            m_ok_button->set_enabled(!m_text_editor->text().is_empty());
+        };
+        m_text_editor->on_change();
+    }
+
     set_rect(x(), y(), max_width + 140, widget->effective_preferred_size().height().as_int());
 }
 

Userland/Libraries/LibGUI/InputBox.h

@@ -14,6 +14,7 @@ namespace GUI {
 
 enum class InputType {
     Text,
+    NonemptyText,
     Password
 };