qt: Keep selection when enabling/disabling passphrase formatting
* qt/pinlineedit.cpp (struct Selection): New. (PinLineEdit::Private::formattedSelection, PinLineEdit::Private::formattedSelection): New. (PinLineEdit::setFormattedPassphrase): Restore adjusted selection after setting formatted/unformatted passphrase.
- GnuPG-bug-id: T5517