QtIFW

Fix MaintenanceTool file write permission check on Linux and macOS

Running the Qt installer on Linux and macOS with sudo will leave
the installation target directory and some installation files
writable by other users than their owner (root). Make
MaintenanceTool check over all subdirectories in the installation
directory for write permission so that admin rights can be always
requested when needed.

Task-number: QTIFW-1324
Change-Id: I9b314853634642b0be1fb3ea7a9164a2d4beb853
Reviewed-by: Katja Marttila <katja.marttila@qt.io>