Replace `~` in backup file path

[droidmonkey]

It would be nice to have expanded evnvar in "Backup destination" as well. I'd like to set that field in one of the following fortmat:

Would that be possible?

Originally posted by @StayPirate in #2406 (comment)

agreed it doesn't work. The error occurs here:

keepassxc/src/gui/DatabaseWidget.cpp

Lines 2087 to 2089 in 64b3695

	if (QDir::isRelativePath(backupFilePath)) {
	backupFilePath = QDir::cleanPath(dbFileInfo.absolutePath() + QDir::separator() + backupFilePath);
	}

[Waqar144]

QFileDialog supports tilde expansion so maybe that can be reused.

QFileDialog f(this, pathWithTilde);
auto expanded = f.directory();

[laur89]

Any traction on this issue? Would be useful also with the URL field on AutoOpen entries -- currently have to hardcode to /home/laur89/file