Allow setting group icons to children groups/entries (#3273)

* Add combo menu button to apply an icon to children

- allow more options to apply icons (child groups, child entries)
- extend tests in TestGroup (applying icons for groups/entries only)
- prevent blue folder icon being set for entries (on entry creation only)

* Do not show the combo menu button for entries

src/fdosecrets/objects/Collection.cpp

@@ -308,7 +308,7 @@ namespace FdoSecrets
             entry->setUuid(QUuid::createUuid());
             entry->setTitle(itemName);
             entry->setUsername(m_backend->database()->metadata()->defaultUserName());
-            group->applyGroupIconTo(entry);
+            group->applyGroupIconOnCreateTo(entry);
 
             entry->setGroup(group);