Lots of changes

"+"
1. Made Model/View scheme working GraphScene is a Model, GraphView is a
View. For other view, which there probably won't be, proxy models will
be created.
2. Moved zooming function to GraphView's slots so they can be called
from actions or whatsoever.
3. Fixed connect button.
4. Fixed remove button with it's memory issue (now edge deletes from
both it's Nodes' lists).
5. Extended Node's name by 1px so it look nicer.
6. If you run the program for the first time you will see welcoming
message.
7. Made a context menu for GraphView. Now it only needs some additions.
8. FileModel* of filesystem moved to GraphScene because of (1)
9. Don't touch on_actionNew_triggered() if you want to use connect() -
later we will firstly check if the graph was changed.
"-"
1. Property model isn't made.
2. Too complex MainWindow::loadSettings() function is from a side of
code-reading.