Same as for main canvas - 2d cursor is aligned to pixels. Visible
difference is different look of antialiased map itself.

Current implementation fills outest pixels incompletely, making slightly
blured look.

When box is moved in scaled state, coordinates get random fraction,
which breaks border-to-pixel alignment.

Make border's outer edge aligned to pixel margin

Mouse buttons in nix world have numbers 1..3 for 3 primary buttons, but
qt has special meaning for numbered names, using symbolics for first
standard buttons.

Simple click removes connections, which are too close to pointer
(for now 1px, but would be nice to make configurable).
Dragging cuts all connections on the way.

Fixes #616

Closes #152

Fixes #632

Fixes #625

Double-bridge is not nice