Code style guidelines

[kaixoo12]

@rodlie @pgilfernandez

Browsing Friction's code reminds me that we should maybe adopt a code style for the project. I like how Inkscape does it:

Bumping the C++ version to something like C++17 or C++20 wouldn't hurt either.

#ifndef CATEGORY_CATEGORY_FILE_NAME_H
#define PRIVATE_DOCUMENT_H

#include "document.h"

#include <stdlib>
#include <QApplication>

#include "../canvas.h"
#include "../memory.h"


namespace Friction::Ui { // Two spaces between headers and code
// No tab for namespaces
class Widget : public Something {
    // People usually agree on 4 spaces, but I prefer something like 8
    // to avoid too much nesting
    public:
         Widget() {};
         ~Widget() = default;
};

}; // namespace Inkscape::Ui

Widget::Widget () {
    if (aSpaceBetweenIsMoreReadable) {
        return;
    }
}

#endif // PRIVATE_DOCUMENT_H