Refactor: Apply layout to *all* interval classif.

Move panel creation up to the PnlAbstractCategorized superclass.
This reduces the amount of code and facilitates maintenance.
Sizing improvements; Default classif: Manual

* Put the field label back and size it to 90 pixels in a WideComboBox
* Make the editor wide enough to avoid unnecessary horizontal scrollbar
* Make the default classification "Manual" again
Put the palette's source in the tooltip.
Make "Field" bold
Move WideComboBox to SIF