1st stage of GUI

Added ability to add vertices and to select them by clicking
in the window. Currently working on creating edges, about
to make changes to the plex.
Edges are being rendered

The bug that caused edges not to show up was fixed.
Hidden edges mechanism is now working properly

Hidden edges allow plex to hold the graph which is not
connected. Two major bugs were fixed:
1. Wrong search algorithm for the suitable parent node.
2. Hidden edge and the real edge were being constructed
	properly, but they were not added to the plex
	at the end, i.e. they were lost.