QtIFW

Fix maintenancetool writing in Windows

In Windows, maintenancetool is written using vb script. Installer
assumed that extension .vbs is always executed with script engine which
is not always true. In such cases create own extension (.qtInstaller)
which can be run with script engine

Task-number: QTIFW-1096
Change-Id: Ic52427e778fc7f3a540256e16b9e67b728c70d41
Reviewed-by: Karsten Heimrich <karsten.heimrich@qt.io>