fix some cosmetic glitches in the codebase

- rename URL into URI in paymentserver where correct
- add some missing Qt-coding-stuff in paymentserver
- change QSpinBox to QLineEdit as base for BitcoinAmountField in .ui files
  (as this is the result when converting the BAF back into base)
- remove some c_str() and replace with QString::fromStdString()
- remove several new-lines
- remove unneeded spaces
- indentation fixes

src/qt/forms/optionsdialog.ui

@@ -461,7 +461,7 @@
  <customwidgets>
   <customwidget>
    <class>BitcoinAmountField</class>
-   <extends>QSpinBox</extends>
+   <extends>QLineEdit</extends>
    <header>bitcoinamountfield.h</header>
   </customwidget>
   <customwidget>

src/qt/forms/qrcodedialog.ui

@@ -186,7 +186,7 @@
  <customwidgets>
   <customwidget>
    <class>BitcoinAmountField</class>
-   <extends>QSpinBox</extends>
+   <extends>QLineEdit</extends>
    <header>bitcoinamountfield.h</header>
   </customwidget>
  </customwidgets>