Remove Gdl and replace it for Gtk.Box'es.

The project gives the following error when it tries to
create a new class fileTab:
	Error building template class 'ValaCATUIFileTab'
	for an instance of type 'ValaCATUIFileTab': Invalid
	object type `ValaCATUIMessageListWidget'
Fixed tabs.

There was a problem including custom templates as object class in
other ui files so I remove this feature and add a parent tab class
with three boxes (center, left and right) the center box has the expand
property to true. In addition I have set the height_request and
width_request properties in some variables.