Allow layer re-ordering in "Layers by Group" tab of Map Definition editor

[jng]

The layers in the "Layers by Group" tab is presented in draw order filtered by their respective parent.

Thus re-ordering layers in this view is still possible as moving a layer up or down in this particular view simply means to move this layer up or down in the draw order to ensure that it above or below the layer we're moving to from within the same group

However, such support for moving layers up or down will only be restricted to within its parent group.