Connectivity graph plot: add basic infrastructure

This commit adds a button named "Open plot" to the connectivity widget
and establishes the basic graph widget infrastructure. The new widget
registers itself as an instance, but not as a skeleton source. The
latter might be added, but is not required for now.

This relates to issue #602.
Connectivity graph plot: draw plots

The bar charts are now drawn in the new widget like they were drawn before in
the connectivity widget. Though, resizing doesn't work, yet.

This relates to issue #602.
Connectivity graph plot: make plots resize automatically

This commit makes the plots resize automatically when the widget is
resized. The width of the graph is the widget's width as long as it
doesn't go below 400px. As height the half of the width is used.

This relates to issue #602.