LibGUI: Register more GML properties and widgets

Register "placeholder" for TextEditor and ComboBox; "models_only"
for ComboBox; Vertical/HorizontalSeparator for SeparatorWidget

Userland/Libraries/LibGUI/ComboBox.cpp

@@ -60,6 +60,9 @@ private:
 
 ComboBox::ComboBox()
 {
+    REGISTER_STRING_PROPERTY("placeholder", editor_placeholder, set_editor_placeholder);
+    REGISTER_BOOL_PROPERTY("model_only", only_allow_values_from_model, set_only_allow_values_from_model);
+
     set_min_width(32);
     set_fixed_height(22);
 
@@ -144,6 +147,16 @@ ComboBox::~ComboBox()
 {
 }
 
+void ComboBox::set_editor_placeholder(const StringView& placeholder)
+{
+    m_editor->set_placeholder(placeholder);
+}
+
+const String& ComboBox::editor_placeholder() const
+{
+    return m_editor->placeholder();
+}
+
 void ComboBox::navigate(AbstractView::CursorMovement cursor_movement)
 {
     auto previous_selected = m_list_view->cursor_index();

Userland/Libraries/LibGUI/ComboBox.h

@@ -60,6 +60,9 @@ public:
     int model_column() const;
     void set_model_column(int);
 
+    void set_editor_placeholder(const StringView& placeholder);
+    const String& editor_placeholder() const;
+
     Function<void(const String&, const ModelIndex&)> on_change;
     Function<void()> on_return_pressed;
 

Userland/Libraries/LibGUI/SeparatorWidget.cpp

@@ -28,6 +28,9 @@
 #include <LibGUI/SeparatorWidget.h>
 #include <LibGfx/Palette.h>
 
+REGISTER_WIDGET(GUI, HorizontalSeparator)
+REGISTER_WIDGET(GUI, VerticalSeparator)
+
 namespace GUI {
 
 SeparatorWidget::SeparatorWidget(Gfx::Orientation orientation)

Userland/Libraries/LibGUI/SeparatorWidget.h

@@ -30,17 +30,43 @@
 
 namespace GUI {
 
-class SeparatorWidget final : public Widget {
+class SeparatorWidget : public Widget {
     C_OBJECT(SeparatorWidget);
 
 public:
     virtual ~SeparatorWidget() override;
 
-private:
+protected:
     explicit SeparatorWidget(Gfx::Orientation);
 
+private:
     virtual void paint_event(PaintEvent&) override;
 
     const Gfx::Orientation m_orientation;
 };
+
+class VerticalSeparator final : public SeparatorWidget {
+    C_OBJECT(VerticalSeparator)
+public:
+    virtual ~VerticalSeparator() override { }
+
+private:
+    VerticalSeparator()
+        : SeparatorWidget(Gfx::Orientation::Vertical)
+    {
+    }
+};
+
+class HorizontalSeparator final : public SeparatorWidget {
+    C_OBJECT(HorizontalSeparator)
+public:
+    virtual ~HorizontalSeparator() override { }
+
+private:
+    HorizontalSeparator()
+        : SeparatorWidget(Gfx::Orientation::Horizontal)
+    {
+    }
+};
+
 }

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -59,6 +59,7 @@ TextEditor::TextEditor(Type type)
     : m_type(type)
 {
     REGISTER_STRING_PROPERTY("text", text, set_text);
+    REGISTER_STRING_PROPERTY("placeholder", placeholder, set_placeholder);
     REGISTER_ENUM_PROPERTY("mode", mode, set_mode, Mode,
         { Editable, "Editable" },
         { ReadOnly, "ReadOnly" },