There is at least an annoyance in the way amount input is presented when sending coins or creating payment requests. I don't really use the GUI so I'm not sure if this happens in other places.

Here is the screenshot of both boxes when inputting the number `15`:
![screenshot from 2018-05-14 10-23-23](https://user-images.githubusercontent.com/5016303/39989540-c9c82bc0-5759-11e8-8341-1981626b1a1b.png)
![screenshot from 2018-05-14 10-23-54](https://user-images.githubusercontent.com/5016303/39989541-c9e95188-5759-11e8-8790-45ebadfbc4c7.png)
**note: just in case somebody wonders, the global dark theme doesn't make a differente on this; same behavior with light/default patterns.*

My **expected behavior** is that, at least, I can see the most significant digits in the amount and, at best, I can see it all when space allows it. 
**What is happening is** that I input `15` and unnecessarily end up seeing `5.00000000`.
The number is complete inside the input as `15.00000000` so it is not really a functionality problem but it can be both annoying and misleading in some cases.

I can reliably reproduce this on **v0.16.0 (and master)** running on a **Fedora 27** using **GNOME 3** and **qt5**.
Tried it on an colleague's OSX and could not reproduce the issue; the input seemed fine.
I'm opening this in case it is an issue for more users.
w += 2; 
            QStyleOptionSpinBox opt;
            initStyleOption(&opt);
            QSize hint(w, h);
            QSize extra(35, 6);
            opt.rect.setSize(hint + extra);
            extra += hint - style()->subControlRect(QStyle::CC_SpinBox, &opt,
                                                    QStyle::SC_SpinBoxEditField, this).size();
            
            opt.rect.setSize(hint + extra);
            extra += hint - style()->subControlRect(QStyle::CC_SpinBox, &opt,
                                                    QStyle::SC_SpinBoxEditField, this).size();
            hint += extra;
            hint.setHeight(h);
            opt.rect = rect();
            cachedMinimumSizeHint = style()->sizeFromContents(QStyle::CT_SpinBox, &opt, hint, this)
                                    .expandedTo(QApplication::globalStrut());
        }
        return cachedMinimumSizeHint;
    }
private:
    int currentUnit;
    CAmount singleStep;
    mutable QSize cachedMinimumSizeHint;
    /**
     * Parse a string into a number of base monetary units and
     * return validity.
     * @note Must return 0 if !valid.
     */
    CAmount parse(const QString &text, bool *valid_out=0) const