Implement a generic EnumValue and a generic EnumValueView

[pfleck]

Creating an Enum involves the creation of the enum-type itself, a ValueTypeValue wrapper and a view for that specific enum. By creating a generic EnumValue a generic EnumValueView can be introduced. This way it is not necessary to create a view for every enum and the generic view can be used instead. The generic view simply contains a dropdown for the given enum, like all the existing enum specific views.

[pfleck]

r12037 Implemented EnumValue and EnumValueView.

[mkommend]

r12148: Changed behavior of EnumValue to avoid subclassing.

[ascheibe]

Reviewed r12148, thanks.

[mkommend]

r12278: Merged r12037 and r12148 into stable.