fix: improve line/arrow resize

[robertwidfen]

Fixes #722

Adds:

• Proper movement of end points
• Snapping of end points to 15° angles with Shift/Control
• Movement of segment by middle handle in direction of segment or perpendicular to it:
  • For vertical/horizontal segment - direction can be switched by Shift/Ctrl
  • For other segments Shift/Ctrl enables this movement and direction is determined by the selected handle

[robertwidfen]

This is more complex than I hoped. In order not to touch all tools there is a new Drawable.resize_bounds_with_segment_target used by line and arrow - without knowing something about the dragged handle I could not get a stable behavior.

Again I hope to have tested all resize cases.