Listeners set before filling fields.
Filler shouldn't enable the combobox.

Form components should be enabled or disabled
by the form logic itself, not by filler components.
Comboboxes can now depend on text fields.

Now the DependentComboboxesHandler accept
textfields as well as comboboxes.
Added method for retrieving all active layers.

Now we can retrieve a list of active layers
through TocLayerManager.