QtIFW

Fix env variable remove on Windows

Removing env variable failed on uninstallation on Windows if the env
variable value had @TargetDir@ in path. File separators got changed
from Windows style to unix style.
Update documentation to inform users to use '\\' as separators when
setting a path as environment variable.

Tested with following values (component.addElevatedOperation(...)
in components.qs):
"@TargetDir@\\lib\\system\\qtplugins"
"not_a_path_but_variable_with_slash_\\qtplugins"
"not_a_path_but_variable_with_slash_/qtplugins"
"%SystemRoot%\\lib\\system\\qtplugins"
"12345"

Task-number: QTIFW-1148
Change-Id: Iaa48d890e70afdbe56bbf389dd4994d5ae91b316
Reviewed-by: Katja Marttila <katja.marttila@qt.io>