Added connect button
Working
1. Added working "Выход" in menu
2. Added working "Новый" in menu
3. Remove now removes selected items and connected to it edges
Merge branch 'develop' of https://github.com/Zo0MER/graph_editor into
develop

Conflicts:
	mainwindow.h
Some simple stuff
Fixed

1. Fixed vanishing of line (overlay)
2. Optimization of line drawing. It no longer takes 100% of CPU when
there is a line on graph.
Changes

1. Made a background grid
2. Deleted unneeded things
3. Fixed wrong splitter alignment, set its' sizes by hard.
4. Hided some headers in treeView