AssistanceIconPanel/RegexAwareComboBox issues

[bobbylight]

This is really two separate issues, but they're kind of related, so I'll fix them at the same time.

1. AssistanceIconPanel listens to a child JTextComponent in a JComboBox, if a combo box is the component being wrapped. On focus gained/lost of the text component, the assistance icon is shown/hidden. However, when the LookAndFeel changes, this child text component can also change. We should update our FocusListener so we keep behaving properly after runtime look and feel changes.
2. When RegexAwareComboBox.setContentAssistImage(Image) is called, we should fire a ContentAssistable.ASSISTANCE_IMAGE event to ensure our displayed icon is properly updated.