Ensured the layout object always holds a reference to the current graph, so it can be save and loaded properly. Fixes #25
Replaced the icon set with simpler countour-only images(closing #11). Also, by default now there is no vertex label text.
1) Added code for loading an image background for the graph (# 12). 2) Added two Hi-Res built-in images (a map of the UK and a world map). 3) Restored the previous (full-color) icon set along with the outline icons (with some slight modifications). The reason is that they look better on the map. 4) Reordered the vertex context menu. 5) Added options to select the background image (including loading from a file) and to change the graph background & edges colors to the general context menu (when right-clicking outside a vertex / edge).
Merge remote-tracking branch 'origin/vladi' into develop