[MAGEMEP-146] Remove filter preventing attributes to show

Make attributes like Meta Keywords to show as well as any system attributes;
only user defined attributes were being displayed on
System/MEP/Profile Editing/Field MappingAdd attribute dialog.
Remove grouping option from dropdown list

Flatten the list of attributes to select from the mapping add attributes dialog.
Merge branch 'MAGEMEP-146' into development
[MAGEMEP-146] Remove grouping option from dropdown list

Flatten the list of attributes to select from the mapping add attributes dialog.
[MAGEMEP-148] Add manage_stock attribute

Attributes from Mage_CatalogInventory need special treatment. When doing the export
it is needed to add them manually.
Merge branch 'MAGEMEP-148' into development
Merge branch 'MAGEMEP-146' into development