QR code fixes

New classes ScanQRTextEdit and ShowQRTextEdit.
Reason: dependencies on zbar availability and issues with the QRTextEdit constructor.
- ScanQRTextEdit needs access to the config (fetch camera). It needs to load
the zbar processor properly before trying to scan. Keeping a reference to
the processor in qrscaner fixes the crashes on windows.
- ShowQRTextEdit should not have access to scan_qr().
- no need to setReadOnly anymore. It is clear from the class name.

Other fixes:
- Show master pub keys now has a Combobox if multiple accounts are available. Pending accounts are filtered out. Selects the text for faster copying.
- Added bitcoin URI parsing on QR code scan
- Fixed copy to clipboard (parent is not always available)