Fix icon selection highlights

- Change QListView mode to ListMode
- Increase custom icon size
- Reduce grid spacing
- Improve selected icon state composite

src/gui/IconModels.cpp

@@ -101,7 +101,6 @@ QModelIndex CustomIconModel::indexFromUuid(const QUuid& uuid) const
     int idx = m_iconsOrder.indexOf(uuid);
     if (idx > -1) {
         return index(idx, 0);
-    } else {
-        return QModelIndex();
     }
+    return {};
 }