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.
{
        CAmount val = 0;
        bool valid = BitcoinUnits::parse(currentUnit, text, &val);
        if(valid)
        {
            if(val < 0 || val > BitcoinUnits::maxMoney())
                valid = false;
        }
        if(valid_out)
            *valid_out = valid;
        return valid ? val : 0;
    }
protected:
    bool event(QEvent *event)
    {
        if (event->type() == QEvent::KeyPress || event->type() == QEvent::KeyRelease)
        {
            QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
            if (keyEvent->key() == Qt::Key_Comma)
            {
                
                QKeyEvent periodKeyEvent(event->type(), Qt::Key_Period, keyEvent->modifiers(), ".", keyEvent->isAutoRepeat(), keyEvent->count());
                return QAbstractSpinBox::event(&periodKeyEvent);
            }
        }
        return QAbstractSpinBox::event(event);
    }
    StepEnabled stepEnabled() const