• CheckBox: Support painting a third state (set client property
  "JButton.selectedState" to "indeterminate").
• TriStateCheckBox component added (see FlatLaf Extras).
• Made JComboBox, JProgressBar, JSpinner and JXDatePicker non-opaque.
  JPasswordField, JScrollPane and JTextField are non-opaque if they have
  an outside focus border (e.g. IntelliJ and Darcula themes). (issues #20 and
  #17)
• Button: Hover and pressed background colors are now derived from actual button
  background color. (issue #21)
• Table: Fixed missing upper right corner (e.g. in SwingX JXTable with column
  control visible).