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.