macdeployqt skips copying if file already exists at destination

If a file already exists in the destination (the app bundle or optional dmg), it will skip recopying it on the second run.

For example, build a project the first time and it will copy all of the needed plugins, qt.conf file, etc, and optionally even create a dmg of the app. If now
some changes to the code is made and and rebuild for second time, None of the files copied during the first time will be copied again. It won't even recreate the dmg file. So the dmg now has the old build version of the app.

At least if an option exists always overwrite files this could be easily fixed.