* Clear previously selected items before adding new ones
* Multiple dispatch emulation by using visitor pattern & default
method in the interfaces:
* geometry/planar/Direction.java
* geometry/planar/Vector.java
* Use ImmutableList<Layer> in board/LayerStructure.java
* layer_list contains all layers (signal & not signal)
* signal_layer_list contains only the signal layers
* Custom ComboBox:
* ClearanceClassBuilder.java
* SignalLayerWithIndexBuilder.java
* ArrayComboBoxModel.java
* Refactor class to enum:
* designformats/specctra/Keyword.java
* Refactor interfaces to class static/abstract methods:
* designformats/specctra/AreaReadable.java
* designformats/specctra/AreaTransformable.java
* designformats/specctra/ShapeReadable.java
* designformats/specctra/ShapeTransformable.java
* designformats/specctra/ShapeWritable.java
* Move static methods to inner classes
* Class structure (organize members)
* New custom exception:
* designformats/specctra/WriteScopeException.java
* Change some access level modifiers to package-private/private
* -h option shows avaliable options
* Use the precalculated width for the components in the right toolbar
* Other improvements
