Add spacer element

Spacer is an element that expands to fill width and height, thus is very
useful in panels to add space between elements.

A possible use is in the tablet user interface in order to align the
indicators to the center.
tablet: Add spacer between elements

Align indicators and launcher to the center.

Issue: #155
tablet: Slightly improve desktop view

Style labels and remove paging because we are not quite ready yet.

Issue: #155