QtIFW

Change ApplicationsDir variable on macOS to return "/Applications"

QStandardPaths::standardLocations(QStandardPaths::ApplicationsLocation)
first element returns $HOME/Applications, as being user writable it
has a higher priority.

Change the variable to return system wide /Applications as it is
a more expected default. Add the user-specific applications
directory as its own variable, and make associated documentation
changes.

Task-number: QTIFW-1011
Change-Id: Ia81a1f5dafdea8665d5f9fc1df3799e59858e5aa
Reviewed-by: Katja Marttila <katja.marttila@qt.io>