Merge plugin.h into Kaleidoscope.h

The plugin.h header only defined the KaleidoscopePlugin class, and while
there was a reason it was separate from Kaleidoscope.h, that reason is
long gone. Merge it there, and remove any reference to plugin.h, as it
is not needed anymore.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

src/Kaleidoscope.h

@@ -22,7 +22,6 @@ void setup();
 
 #include KALEIDOSCOPE_HARDWARE_H
 #include "key_events.h"
-#include "plugin.h"
 #include "layers.h"
 
 extern HARDWARE_IMPLEMENTATION KeyboardHardware;
@@ -33,6 +32,11 @@ extern HARDWARE_IMPLEMENTATION KeyboardHardware;
 
 #define KEYMAP_SIZE (sizeof(keymaps) / ROWS / COLS / sizeof(Key))
 
+class KaleidoscopePlugin {
+ public:
+  virtual void begin(void) = 0;
+};
+
 class Kaleidoscope_ {
   public:
     Kaleidoscope_(void);

src/layers.h

@@ -2,7 +2,6 @@
 
 #include <Arduino.h>
 #include "key_defs.h"
-#include "plugin.h"
 #include KALEIDOSCOPE_HARDWARE_H
 
 class Layer_ {

src/plugin.h

@@ -1,7 +0,0 @@
-#pragma once
-
-class KaleidoscopePlugin {
-  public:
-    virtual void begin(void) = 0;
-};
-