Allows to style ToolItems background color and foreground color.
SWT style-bits can also be used for selecting the tool items which
should be styled.

ToolItemTest contains tests for these elements, an TODO has been added
so enable the styling also via a styling call on the parent, this would
require additional changes in ToolBarElement to handle also its
children.

Pseudo-handing styling similar to the button, e.g. :checked has not yet
been implemented.

ToolItem {
   background-color: pink;
   color:blue;
}

ToolItem [style~='SWT.PUSH'], ToolItem[style~='SWT.CHECK'] {
   background-color: blue;
   color:red;
}