Installer: Fix "an" -> "and" typo in code comments

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Installer: Move DeleteFromVirtualStore to helpers.inc.iss

As it is more or less a generic helper function.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Installer: Improve code comments

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Installer: Reduce code duplication by refactoring into functions

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Installer: Minor equalization of formatting

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>