Objects' topology helper library

"Topohelp" == "Topology helper," but I'm open to a better name 😄

This is just the library base containing only the connection-creation-related code from the graphicsviewutils, with small fixes and improvements based on clang's hints.

Internally it splits the code scopes using namespaces, and I'd want to do a similar thing on the level of includes to use it as: ~~

#include "topohelper/connection.h"

But I don't want to have a folder structure as spacecreator/src/libs/topohelper/topohelper. Any ideas if we can do this poor by CMake?

UPD:

• include paths corrected to be used as #include "topohelper/connection.h";
• All the functions from graphicsviewutils moved to topohelper except those directly related to QGraphicsScene/rendering things;
• Unused methods removed;
• Only-internally-used functions are not exposed (made "private");
• Documentation actualized (mostly);
• distanceLine/Polygon renamed to calcDistance;
• calcDistance uses QLineF::length.