Add easy way to create installer who remove old application version before instaling new one.

Many people ask me "if Qt Installer Framework can update the application so that it will first uninstall the previous version and then install the new one" ?

Example.
1. myinstaller-1.0 create myapp-1.0.exe
2 myinstaller-2.0 remove all files of myinstaller-1.0 and create myapp-2.0.exe

I said it is possible, but it's complicated because it requires some changes in controlscript.js

—
Disadvantages of running uninstaller in control script:

1. The user has to enter the administrator password multiple times

2. The uninstaller deletes files with a delay, even after the uninstaller has finished, you can still see the deleted files. This causes the problem that we don't know when the previous version uninstaller finished. It is possible that we start installing new files while deleting the old files is not finished yet

–

This is a very important point when choosing an installer for your application