[gui] visual "overflow" of amount input #13231

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: **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.

The amount spin box (custom class) has setAlignment(Qt::AlignRight); which could be switch to left... The field has also a max length of amount->setMaximumWidth(170); which could be increased.

Hello, I'd like to work on this! I'll try having a PR ready in a few days.

Sure, go on! Thanks.