#28 don't limit positioning of graphical elements

[Knut Franke]

Submitted via mail by Semen Esilevsky.

Legends, arrows, text etc. should be allowed outside the box defined by
the graph axes (now they are cropped by this box). Imagine the plot with
20+ curves, which fill all the space. Where to place a legend? Right,
outside the box.

As suggested by Tilman, we're planning to use an approach similar to vector drawing applications for all elments in a Graph (which will probably be renamed to Worksheet, following LabPlot terminology) instead of the current widget-layout based one.