• Use new chevron arrows in "Flat Light" and "Flat Dark" themes, but keep
  triangle arrows in "Flat IntelliJ" and "Flat Darcula" themes. (issue #7)
• Use bold font for default buttons in "Flat IntelliJ" and "Flat Darcula"
  themes.
• Hide label, button and tab mnemonics by default and show them only when
  Alt is pressed. (issue #4)
• If a JButton has an icon and no text, then it does not get a minimum width
  (usually 72 pixel) and the left and right insets are same as top/bottom insets
  so that it becomes square (if the icon is square).
• Changed styling of default button in "Flat Light" theme (wide blue border
  instead of blue background).
• Added Java 9 module descriptor module-info.class to flatlaf.jar (in
  META-INF/versions/9). But FlatLaf remains Java 8 compatible. (issue #1)
• Support specifying custom scale factor in system properties flatlaf.uiScale
  or sun.java2d.uiScale. E.g. -Dflatlaf.uiScale=1.5. (Java 8 only)