Add a build option to not include the example styles in the binary

For most projects we are currently working on it's important for the customer to use the vanilla version of the virtualkeyboard (as well for other repositories), but they want to integrate it into their custom ui, so they can't use the built-in style which comes with the virtualkeyboard. Currently there is no way to disable the built-in style so it's always packed and included into the plugin altough it's never been used for this application. I think it would be great if we could haven an option (similar to the options for disabling/enabling certain layouts) to disable the built-in style.