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/gui/group/EditGroupWidget.cpp

@@ -212,6 +212,17 @@ void EditGroupWidget::apply()
     // Icons add/remove are applied globally outside the transaction!
     m_group->copyDataFrom(m_temporaryGroup.data());
 
+    // Assign the icon to children if selected
+    if (iconStruct.applyTo == ApplyIconToOptions::CHILD_GROUPS
+        || iconStruct.applyTo == ApplyIconToOptions::ALL_CHILDREN) {
+        m_group->applyGroupIconToChildGroups();
+    }
+
+    if (iconStruct.applyTo == ApplyIconToOptions::CHILD_ENTRIES
+        || iconStruct.applyTo == ApplyIconToOptions::ALL_CHILDREN) {
+        m_group->applyGroupIconToChildEntries();
+    }
+
     setModified(false);
 }