OxideSubFrame.java: added and implemented.

OxideFrame.java: added border by adjusting decoration pane and content
pane position and defining the background color for the frame's root
content pane.

TestSubFrame.java: added and implemented an exhibition OxideSubFrame.

close_icon_18x18.png, minimize_icon_18x18.png: added these icons that
accompany the OxideSubFrame decoration.