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.
QWidget(parent),
    amount(0)
{
    amount = new AmountSpinBox(this);
    amount->setLocale(QLocale::c());
    amount->installEventFilter(this);
    amount->setMaximumWidth(170);
    QHBoxLayout *layout = new QHBoxLayout(this);
    layout->addWidget(amount);
    unit = new QValueComboBox(this);
    unit->setModel(new BitcoinUnits(this));
    layout->addWidget(unit);
    layout->addStretch(1);
    layout->setContentsMargins(0,0,0,0);
    setLayout(layout);
    setFocusPolicy(Qt::TabFocus);
    setFocusProxy(amount);
    
    connect(amount, SIGNAL(valueChanged()), this, SIGNAL(valueChanged()));
    connect(unit, SIGNAL(currentIndexChanged(int)), this, SLOT(unitChanged(int)));
    
    unitChanged(unit->currentIndex());
}
void BitcoinAmountField::clear()