QtIFW

Make communication via installer.execute() Unicode safe

It was impossible to pass Unicode data safely to a process started via
installer.execute(), or to read Unicode data printed by that process
safely back in. The reason for this is that the code hardcoded the
latin1 codec for converting between strings used in the script
interpreter and bytes used by the QProcessWrapper API.

Fix this by adding two new optional arguments to installer.execute()
which can be used to define the codec to be used for writing to stdin
resp. reading from stdout. This defaults to latin1 for backwards
compatibility.

Change-Id: I290d8d9617b286ef90b2f0a05c6e7a47f6df317f
Reviewed-by: Karsten Heimrich <karsten.heimrich@theqtcompany.com>
Reviewed-by: Frerich Raabe <raabe@froglogic.com>