Rename widget class hierarchy

[ctrueden]

InputWidget -> Widget
InputPanel -> WidgetPanel
AbstractInputWidget -> AbstractWidget
AbstractInputPanel -> AbstractWidgetPanel
etc.
javadoc: "input widget" -> "UI widget"
javadoc: "input panel" -> "widget panel"
javadoc: "input type" -> "parameter type" ?

Ultimately, Widgets should be usable outside of InputHarvester, too.

Migrated-From: http://trac.imagej.net/ticket/1698

[hinerm]

Refactor SwingChoiceWidget and SwingObjectWidget to extend common AbstractSwingComboWidget.