UI: Increase hitboxes and interaction areas for core elements

[edumeneses]

Closes #1994

Increased hitboxes and interaction areas for various UI components to improve selection and drag-and-drop usability:

• Cables: Increased stroke width from 7 to 14.
• Intervals: Added a 5-pixel margin and increased drop threshold to 12.
• Conditions: Increased stroke width from 1 to 10.
• Events: Widened bounding box from 6 to 12.
• States: Enlarged visual radii (full: 6->8, point: 3.5->5).
• Ports: Expanded interaction area to 20x20.
• Slot Headers: Widened drag area and increased drop threshold between slots.

[jcelerier]

works here! but for me it seems to make it harder to select the cross to create new objects in the timeline: basically only the lower-right or upper-left corner triggers the "creation" yellow or red plus signs now :/

ossia-.2.mp4

[jcelerier]

the correct thing to do here in addition to this change is: in src/plugins/score-plugin-scenario/Scenario/Document/State/StateView.hpp, we should implement the following virtual methods for QGraphicsItem, so that it knows that the item does not only have a square shape:

virtual QPainterPath opaqueArea() const;
virtual bool contains(const QPointF &point) const;
virtual QPainterPath shape() const;

an example is in src/plugins/score-plugin-curve/Curve/Segment/CurveSegmentView.hpp.

Generally most items should implement these virtual methods, it's more work but also means that if you click outside of the visible area it will be correct