[spark-def] FxButton (and FxToggleButton, etc) should support icons

Wonderful prose:
The Gumbo FxButton and FxToggleButton should support an icon style.

I figure the easiest (possibly most efficient) solution would be if we included an extra skin in the spark skins folder which would position an icon on the left of the label (if users want to position it to the right/top/bottom, they can copy/paste our skin and position it however they please).

Also, this would me do hella-awesome state's specific icon stuff like:

FxButton:up {
icon: @Embed("upIcon.png");
}
FxButton:upAndSelected {
icon: @Embed("upAndSelectedIcon.png");
}
FxButton:disabled {
icon: @Embed("disabledIcon.png");
}
.confirmButton:up,
.confirmButton:over,
.confirmButton:down {
icon: @Embed("confirmIcon.png");
}

...etc.

We could/should leave the non-icon skin as the default, as it should be easy to override the default behavior with something like:

FxButton {
skinClass: ClassReference("mx.skins.spark.FxIconButtonSkin");
}