#538 Proper button margins for Nimbus L&F

[Makarius]

Nimbus buttons have an empty border by default (according to getBorder/setBorder), but there is an extra "content border" for painting.

See also http://stackoverflow.com/questions/8764602/how-to-override-nimbus-button-margins-for-a-single-button

The patch uses the described technique to adjust the borders for the dockable window panel accordingly, such that Nimbus is no longer deformed.

Note that the Insets(6, 6, 6, 6) stems from Button.contentMargins in javax/swing/plaf/nimbus/NimbusDefaults.java