Reintroduce "Read settings file for loading external tutorials"

This reintroduces commit 29a4c9bfdacd3c721f47abc764efc6db87533c66,
temporarily reverted prior to mer-3.1 merge with commit
832935a2115234082bace8a5e363991d43141d0d.

Note that the setting keys are now Help/InstalledExamples and
Help/InstalledTutorials - see 29b0c10 (WelcomePage: Support additional
(non-qt) example sets)

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>