[systemsettings] Import changes from Jolla.

TBD: Make this a library. This is required for properly unforking this from the
Jolla side, and will be done in the next commit.
[systemsettings] Seperate code into a shared library and plugin.

Also ports this to Qt 5, without worrying about Qt 4 compatibility -- as Qt 4 is
going away.
Merge pull request #1 from rburchell/master

Work at unforking systemsettings from Jolla.