Added Theme Singleton for changing theme in runtime.

Also modified example with usage of PageCurl effect for
changing themes.

Change-Id: I5747d5a3450abe9e01c1939ec4129d30b244544d
Reviewed-by: Tomasz Olszak <olszak.tomasz@gmail.com>
ToolBarStyle - removed software buttons.

Also toolBar.backAction now triggered when Back HW button
released or ESC released.

Change-Id: I50c1eebf0c0df62cde65239bbfc77829b416be4a
Reviewed-by: Jarosław Staniek <staniek@kde.org>