virtualkeyboard: rename class to InputMethod

VirtualKeyboard class does not implement the relevant VirtualKeyboard protocol but rather implements the InputMethod protcol and can in theory be used by other input method like e.g. ibus.

Make class name consistent with what it does to avoid confusion in future.

For now only rename of main class is done and dbus service is kept as-is to provide retro-compatibility, when input method protocol is implemented fully, we can think of what to do wrt the dbus interface later when we fully implement zwp_input_method_unstable_v1 protocol.