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

tests/TestGroup.h

@@ -47,6 +47,7 @@ private slots:
     void testEquals();
     void testChildrenSort();
     void testHierarchy();
+    void testApplyGroupIconRecursively();
 };
 
 #endif // KEEPASSX_TESTGROUP_H