Refactor drawing of elements: hide logic behind common interface

[wawuwo]

Hi!

This is yet another bit of refactoring. The core idea is to create a common interface for such primitives like qucs::Line, qucs::Arc, qucs::Area and to hide drawing logic behind the interface.

Look at this, where client has to select a proper method of QPainter and deal with its arguments not its own:

        for (qucs::Area *pa: Rects) {
            p->setBrush(pa->Brush);
            p->drawRect(cx + pa->x, cy + pa->y, pa->w, pa->h);
        } 

— next time when rect is drawn everything has to be done again, and most likely will be just copied and pasted, making duplicate.

And compare with this:

        for (qucs::DrawingPrimitive* rect : Rects) {
            p->setBrush(rect->brushHint());
            rect-draw(p, cx, cy);
        } 

— where

• drawing logic (drawRect(cx + pa->x, cy + pa->y, pa->w, pa->h)) is hidden and defined only once in member function body
• it's cleaner
• you can drawn different kinds of primitives all in one loop, because they have the same interface

To achieve this I did the following steps:

1. Created a new type for ellipses. Previously ellipses and rectangles were defined by the same type Area and client had to decide somehow what this area is — an ellipse or a rectangle
2. Created a common interface qucs::DrawingPrimitive for qucs::Line, qucs::Arc, qucs::Rect, qucs::Ellips
3. Replaced all approach to drawing of primitives with invocations of new API.

---

Everything should look the same, I haven't found any issues using the changed build on my machine.

[zergud]

Looks good

[wawuwo]

I'm sorry I had to turn it into draft, it feels like it went in a wrong direction. I'll try to find a way to make it better

[wawuwo]

Closed in favour of #723