TabbedPane custom components on left and right sides of tabs area

[DevCharly]

This PR introduces support for adding custom components to left and right sides of tabs area.

Set client properties JTabbedPane.leadingComponent or JTabbedPane.trailingComponent on a JTabbedPane to add components:

myTabbedPane.putClientProperty( "JTabbedPane.leadingComponent", new JButton( "Hello" ) );
myTabbedPane.putClientProperty( "JTabbedPane.trailingComponent", new JButton( "World" ) );

The preferred width of the added components is used, but the height is always set to the tab height.
Use an additional panel if your component should have a smaller height.

Edit: since commit dc92d09 the trailing component occupies all available horizontal space, which allows you to use all space on the right side of the tabs:

There is no gap between the leading/trailing components and the tabs.

If you want place buttons into the tab area, it is recommended to use a JToolBar:

(code from FlatLaf Demo)

JToolBar leading = new JToolBar();
leading.setFloatable( false );
leading.setBorder( null );
leading.add( new JButton( new FlatSVGIcon( "com/formdev/flatlaf/demo/icons/project.svg" ) ) );
myTabbedPane.putClientProperty( "JTabbedPane.leadingComponent", leading );

Because the trailing component occupies all available space, it is necessary to add a horizontal glue with javax.swing.Box.createHorizontalGlue() to the toolbar to get right aligned buttons:

JToolBar trailing = new JToolBar();
trailing.setFloatable( false );
trailing.setBorder( null );
trailing.add( new JButton( new FlatSVGIcon( "com/formdev/flatlaf/demo/icons/buildLoadChanges.svg" ) ) );
trailing.add( Box.createHorizontalGlue() );
trailing.add( new JButton( new FlatSVGIcon( "com/formdev/flatlaf/demo/icons/commit.svg" ) ) );
trailing.add( new JButton( new FlatSVGIcon( "com/formdev/flatlaf/demo/icons/diff.svg" ) ) );
trailing.add( new JButton( new FlatSVGIcon( "com/formdev/flatlaf/demo/icons/listFiles.svg" ) ) );
myTabbedPane.putClientProperty( "JTabbedPane.trailingComponent", trailing );

This is part of TabbedPane improvements as discussed in issue #40 (comment).

CC @smileatom @Chrriis