Fix hovering, selection, dragging, and middle-click box selection bugs

[potrepka]

The following are possible actions (bugs) that exist in the current codebase:

• You can hover over nodes, links, and pins while mouse is outside of canvas
• You can select nodes and links while mouse is outside of canvas
• You can begin dragging nodes while mouse is outside of canvas
• You can interact with pins (creating a link) while mouse is outside of canvas
• You can hover over nodes, links, pins while performing a box select
• You can hover over pin and node at the same time
• You can hover over pin and link at the same time
• When simulated middle-click is enabled, you can begin a box selection with middle-click that does not finish when middle mouse button is released

This pull request fixes all of these issues. Please let me know if you would like me to make any changes to the pull request.

[Nelarius]

I'd be happy to merge the change now -- do you want to apply the fix you mentioned to this pr, or do that in a separate pr?

[potrepka]

@Nelarius I’m ready to merge this pr. I will fix the other things I mentioned in a different pr.