reasoning identic to
commit 8f8e8a2e06
adapt KVComboBoxes to pass store items using 'comboItems' parameter
commit f2782813cd
ext6migrate fix model behaviour for KVComboBox
our parent class Ext.grid.GridPanel sets 'tbar: null'
and Ext.applyIf() will only apply our property
if it is undefined, so switch to Ext.apply()
it is safe to use Ext.apply() because none of these classes is further extended,
so we don't have to worry that might me might override a toolbar
set further down by a child class
this fixes allows the panel to display its top toolbar with add/edit buttons