BB10: Better support for dark theme

Dark theme is default on devices with OLED display and some resources
where not visible due to being dark too.
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