add the possibility to draw rectangles behind vertex labels to improve readability

[elbosso]

I propose to attach not only a label to each vertex but a rectangle as well - this is then painted underneath the label, adding visual distinction and improving readability.

For example:

[elbosso]

to this end, i propose a new style class (example):

.vertex-label-rect {
    -fx-stroke-width: 3;
    -fx-stroke: #A0A0A0;
    -fx-stroke-type: outside;
    -fx-fill: #D0D0D0;
    -fx-opacity: 0.4;
}