ISIS-478: better handling of primitives as choices

* ObjactActionParameterAbstract's check for compatibility of returned choices vs parameter types now treats wrapper and primitive types (eg int.class and java.lang.Integer.class) as compatible
* AdapterInvokeUtils#invokeAutoFit will substitute default values for primitives (eg 0 for an int), rather than null (which fails, obviously)

In addition:
* removed some duplication between ValueChoicesSelect2Panel and EntityLinkSelect2Panel (use of ObjectAdapterMementoProvider for choices provider impl)
* working towards EntityLinkSelect2Panel supporting onChange [fixed] and correct handling of pending values for onUpdate [not yet fixed]
ISIS-478: done (EntityLinkSelect2Panel working)

* EntityLinkSelect2Panel now also supports dynamic choices (as well as ValueChoicesSelect2Panel, previously)

in addition:
* factored out common code with ValueChoicesSelect2Panel(the ScalarModelWithPending interface)
* split EntityLinkSelect2Panel into another, simpler, panel - EntityLinkSimplePanel, just for showing icon/links