Add async support for installer.execute

When installing QT with the official installer, the first page/step is to login to Qt. When you enter your details and click next, you are presented with a progressbar and the next button is disabled while that authentication operation occurs in the background.

There doesn't seem to be a way to replicate this using the vanilla installer framework. The closest equivalent I can see is to use `installer.execute(<script to do networking>)` , but this will block the UI thread of the installer, such that any custom progress bar will appear frozen.

This could be achieve by adding better support, through signals or otherwise to executeDetached