OxideMenuButton.java: changed mouse hovering behavior such that the
button's background color will change when the mouse is hovering over
it.

OxidePalette.java: added the menuButtonFontColor constant to define the
color of OxideMenuButton instances' label text.

OxideFrame.java: set the opacity of the minimize and exit frame control
bottons to transparent so their background colors don't change when the
mouse cursor hovers over them.