Add basic infrastructure for settings widget
Settings widget: add infrastructure for adding content to settings widget

This allows to add single blocks that can be collapsed and expanded. This might
be useful to hide unrelated settings.
Fix critical bug view box creation function of stack

Fortunately, this method isn't used so far. It will, however, be used in the
grid layer that I work on in this branch. The problem was that the Y information
of the view box wasn't calculated correctly.
Grid layer: add basic grid drawing functinality

The layer can now be added with the help of the settings widget. At the moment,
it draws cells of a fixed 1nm size in each dimension. If activated, it is added
to every open stack.

This targets issue #646.
Grid layer: add basic settings user interface

This is allows the adjustment of all basic grid settings (cell width, cell
height, x offset, y offset) from the settings widget.

This targets issue #646.
Grid layer: make settings look a bit more aligned
Grid layer: add line width setting
Grid layer: allow options to be passed to constructor

This allows setting options before the grid is enabled. When enabled, the new
grid will have the options set right away.
Merge branch 'grid-overlay'