DisplaySettings: Allow setting wallpapers from outside /res/wallpapers

Ideally, we would copy the file to `/res/wallpapers`, add an entry to
`ComboBox` model and set the index to this entry. I didn't want to
touch anything outside of `DisplaySettings`, so this is more of a
workaround :^)

Applications/DisplaySettings/DisplaySettings.cpp

@@ -125,6 +125,11 @@ void DisplaySettingsWidget::create_frame()
     m_wallpaper_combo->set_model(*ItemListModel<AK::String>::create(m_wallpapers));
     m_wallpaper_combo->on_change = [this](auto& text, const GUI::ModelIndex& index) {
         String path = text;
+        if (m_monitor_widget->set_wallpaper(path)) {
+            m_monitor_widget->update();
+            return;
+        }
+
         if (index.row() == 0) {
             path = "";
         } else {
@@ -136,8 +141,8 @@ void DisplaySettingsWidget::create_frame()
             }
         }
 
-        this->m_monitor_widget->set_wallpaper(path);
+        m_monitor_widget->set_wallpaper(path);
-        this->m_monitor_widget->update();
+        m_monitor_widget->update();
     };
 
     auto& button = wallpaper_selection_container.add<GUI::Button>();

Applications/DisplaySettings/MonitorWidget.cpp

@@ -33,10 +33,14 @@ MonitorWidget::MonitorWidget()
     m_monitor_rect = { 8, 9, 320, 180 };
 }
 
-void MonitorWidget::set_wallpaper(String path)
+bool MonitorWidget::set_wallpaper(String path)
 {
     m_desktop_wallpaper_path = path;
-    m_desktop_wallpaper_bitmap = Gfx::Bitmap::load_from_file(path);
+    auto bitmap_ptr = Gfx::Bitmap::load_from_file(path);
+    if (!bitmap_ptr)
+        return false;
+    m_desktop_wallpaper_bitmap = bitmap_ptr;
+    return true;
 }
 
 String MonitorWidget::wallpaper()

Applications/DisplaySettings/MonitorWidget.h

@@ -35,7 +35,7 @@ class MonitorWidget final : public GUI::Widget {
 public:
     MonitorWidget();
 
-    void set_wallpaper(String path);
+    bool set_wallpaper(String path);
     String wallpaper();
 
     void set_wallpaper_mode(String mode);