Grouped window list: Spacing is defined by CSS

[AxXxB]

Adding ability to adjust a spacing between app groups via theme CSS. This is done as in window list applet but using following classes:

/* ===================================================================
 * Grouped window list ([email protected])
 * ===================================================================*/
.grouped-window-list-box {
    spacing: 2px;
}
.grouped-window-list-box.vertical {
    spacing: 3px;
}

If there is no spacing defined in CSS, the old value of 6px is used.

Related to #10407

[mtwebster]

Hi can you squash this into a single commit? It's a small enough set of changes, and all for addressing the same issue.

Thanks

[AxXxB]

@mtwebster Sure! Done.