* Add initial printing support with QtWebEngine >= 5.7.0
This works by printing to PDF first, then sending the PDF to a printer
if printing to a physical printer was requested.
On *nix (including Mac OS X), we use the lpr command, which is typically
provided by CUPS nowadays. Currently, no options beyond the printer name
are passed, there is room for improvement there. (Okular has a
FilePrinter class that handles this in a more sophisticated way.)
On Windows, we use ShellExecuteW with the "printto" verb. In that case,
the printer name is unfortunately the only thing that can be passed. The
user experience may also be suboptimal depending on the PDF viewer
installed on the system. (E.g., Adobe Reader is reported to ignore
SW_HIDE.) And if there is no PDF viewer installed at all, it will of
course not work at all. But it is the best we can do without bundling
something like GhostScript (or requiring the user to install it).
* Printing: Let lpr autoremove the temporary file instead of QTemporaryFile
Only lpr knows for sure when it is safe to delete the temporary file.
* Printing: Use the callback version of printToPdf instead of the file one
The file version is also asynchronous and does not report when it is
done.
* Printing: Use async QProcess API instead of QProcess::execute to run lpr
This also required making tempFile a QTemporaryFile * instead of a
QTemporaryFile on the stack, because QTemporaryFile is a QObject and
cannot be copied.
* Printing: Adapt the FilePrinter from Okular to pass correct lpr args
The license of those files is GPLv2+, which is compatible with
QupZilla's GPLv3+ licensing.
