Add LE support to manual bttestui application

The bttestui application is a manual user-clickable test application for testing Bluetooth features in a manual way and allows quick and point-like isolated testing for multiple purposes. This is often helpful during development time, or when a change's test cannot be automated.

The current bttestui is limited to classic bluetooth. This task is about adding some basic level LE functionality into the application. This includes at least things such as advertising, discovery, (dis)connection, read/write.