1) Moved all layout-related fields of the graph to a new 'LayoutParameters' class and slightly changed the mechanic behind calculating the predominant VertexIcon. Also fixed a bug where the predominant icon was not updated when adding or removing vertices. 2) The graph position and zoom level is now reset every time a new graph is generated. 3) Vertex positions for lattice-type graphs are now calculated in such a way, so the entire graph is fitted in the initially visible visualization space. There is also an option in the lattice generation settings to scale the generated layout (the default being 100%). This closes #40. 4) Set a limit to the minimum and maximum zoom levels that can be set to the graph view. Also, pressing the middle mouse button now switches the zoom mode (direction) between Windows and MacOS style.