Make KaleidoscopePlugin.begin protected by default

Make `Kaleidoscope_` a friend class, so that it can access `.begin`. The reason
behind this is that `.begin` is an interface towards `Kaleidoscope.use()`, and
that function should be the only user. To discourage its use, make it protected.

This does not break any existing - and valid - code, but allows us to slowly
migrate the plugins to a protected `begin()` method.

Fixes #177.

Signed-off-by: Gergely Nagy <kaleidoscope@gergo.csillger.hu>

src/Kaleidoscope.h

@@ -37,8 +37,11 @@ extern HARDWARE_IMPLEMENTATION KeyboardHardware;
 
 #define USE_PLUGINS(plugins...) Kaleidoscope.use(plugins)
 
+class Kaleidoscope_;
+
 class KaleidoscopePlugin {
- public:
+  friend Kaleidoscope_;
+ protected:
   virtual void begin(void) = 0;
 };