QtIFW

Gain admin rights for writing MaintenanceTool config files if needed

Running MaintenanceTool from an elevated directory didn't do proper
checks for file writing rights when parsing Updates.xml from a
repository, for example when a default repository contains actions
inside <RepositoryUpdate> tags the MaintenanceTool couldn't write
changes to the .ini file immediately.

Make MaintenanceTool do proper rights checking when parsing Updates.xml.
Create uniform methods for checking directory writing access as it is
used on several occasions. Add tests for directory checking methods.

Task-number: QTIFW-1363
Change-Id: I6d634ddbd59429e5b98af1d8010e70a71de40f69
Reviewed-by: Katja Marttila <katja.marttila@qt.io>