Improve Upload Field implementation

Current solution involves composite field based solution and requires the presence of redundant static fields for dimensions (width and height) and file name. Those are quirky implementation-wise and also are buggy.

The following improvements should be made to the Upload Field implementation:

• Attach all metadata information that is part of a File to the UploadField (removing the need for redundant static fields).
• Add required property and validation to UploadField
• Enable adding custom factoryClass for the UploadField to maintain flexibility in adding new features to the UploadField (DamUploadField in Assets app).
• Remove redundant static fields from Assets app (and update the app descriptor for the improved UploadField) 
• Extract ValueBoundProperty interface from info.magnolia.ui.field.FieldDefinition, which enables adding a Value based property trait to both simple FieldDefinition (which always have a value) or a info.magnolia.ui.editor.ComplexPropertyDefinition (which may or may not have a Value trait).