Black theme works correctly

Still needs to make theme selectable
Added switch for changing between white and black themes

Using the switch doesn't do anything yet.
Can now change between black and white themes on the settings page

The theme setting is not saved yet.
Default to white theme.
White theme verified to still look how should be.
Remembers the black/white theme setting
Added functions needed to remember the set in use to the C++ side

Also added SettingsHandler to MainPage.qml. Not used yet.
Remembers the last used number set
Select a number set dialog has the current set preselected