BB10: A small fix for BlackBerry 10.3

In BlackBerry 10.3 title in white theme is not branded anymore so title
text is badly visible (white text on light background).
New AppSettings and SingleSettings classes

SingleSettings implements singletone QSettings instance while
AppSettings will provide enhanced QSettings functionality to C++ and
QML (namely, signals when settings value is changed).
Switch C++ code to use new AppSettings class
Implementation of QML side of AppSettings class
WIP: Settings