Examples must set a non-native style to get a consistent look

The Controls native style work causes issues in Quick 3D examples:

After building the latest qtdeclarative and qtquickcontrols2:

• some examples now use the native style, leading to sub-standard look,
• others sometimes use the non-native one (sliders for example), while some controls (checkboxes) get a mixed native + non-native look
  • this is due to setting the 'background' delegate, but not 'foreground' in our somewhat customized controls in some of the examples

Suggest we solve this by hardcoding the Basic style in main.cpp for all the examples.