elementary: move definition of Elm_Gen_Item_Class to elm_general.eot

elementary: move definition of Elm_Gen_Item_Class to elm_general.eot

Move the definition of Elm_Gen_Item_Class from elm_gen.h to
elm_general.eot.
Also move the definition of its aliases Elm_Gengrid_Item_Class and
Elm_Genlist_Item_Class from elm_gengrid_common.h and
elm_genlist_common.h to elm_general.eot.

Rename these types to Elm.Gen.Item.Class, Elm.Gengrid.Item.Class and
Elm.Genlist.Item.Class to follow Eolian standards. And rename all
occurrences in .eo files accordingly.

Put elm_gen.h include above elm_general.h to allow the callback fields
to be defined in elm_general.eot.