qt: Support building with Qt 5.9
* qt/pinlineedit.cpp (class PinLineEdit::Private): Add field q. (PinLineEdit::Private::Private): New. (PinLineEdit::Private::copyToClipboard): Remove obsolete parameter. Use new field q instead. (PinLineEdit::Private::selectionEnd): New. (PinLineEdit::PinLineEdit): Pass this to Private. (PinLineEdit::setFormattedPassphrase): Use new selectionEnd. (PinLineEdit::copy): Call copyToClipboard without parameter.
QLineEdit::selectionEnd() exists since Qt 5.10. Provide an alternative
implementation for Qt 5.9 and earlier. This makes it possible to build
pinentry-qt on CentOS 7 which is used for building an AppImage.
- GnuPG-bug-id: T5592