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