LibGUI: Make button icons settable in GML

`Widget::load_from_gml()` doesn't yet return `ErrorOr`, so we log a
warning message if loading the icon fails.

Userland/Libraries/LibGUI/Button.cpp

@@ -29,6 +29,8 @@ Button::Button(String text)
         "button_style", button_style, set_button_style, Gfx::ButtonStyle,
         { Gfx::ButtonStyle::Normal, "Normal" },
         { Gfx::ButtonStyle::Coolbar, "Coolbar" });
+
+    REGISTER_STRING_PROPERTY("icon", icon, set_icon_from_path);
 }
 
 Button::~Button()
@@ -140,6 +142,16 @@ void Button::set_icon(RefPtr<Gfx::Bitmap>&& icon)
     update();
 }
 
+void Button::set_icon_from_path(String const& path)
+{
+    auto maybe_bitmap = Gfx::Bitmap::try_load_from_file(path);
+    if (maybe_bitmap.is_error()) {
+        dbgln("Unable to load bitmap `{}` for button icon", path);
+        return;
+    }
+    set_icon(maybe_bitmap.release_value());
+}
+
 bool Button::is_uncheckable() const
 {
     if (!m_action)

Userland/Libraries/LibGUI/Button.h

@@ -21,6 +21,7 @@ public:
     virtual ~Button() override;
 
     void set_icon(RefPtr<Gfx::Bitmap>&&);
+    void set_icon_from_path(String const&);
     const Gfx::Bitmap* icon() const { return m_icon.ptr(); }
     Gfx::Bitmap* icon() { return m_icon.ptr(); }