Add static method button::process_all()

This is a convenience method that simply calls process() for each of the
EV3 buttons. See #90.

cpp/ev3dev.cpp

@@ -35,6 +35,7 @@
 #include <fstream>
 #include <list>
 #include <map>
+#include <array>
 #include <algorithm>
 #include <system_error>
 #include <mutex>
@@ -916,6 +917,7 @@ bool button::process()
 }
 
 //-----------------------------------------------------------------------------
+
 #ifndef NO_LINUX_HEADERS
 button button::back (KEY_BACKSPACE);
 button button::left (KEY_LEFT);
@@ -924,6 +926,21 @@ button button::up   (KEY_UP);
 button button::down (KEY_DOWN);
 button button::enter(KEY_ENTER);
 #endif
+
+//-----------------------------------------------------------------------------
+
+bool button::process_all() {
+  std::array<bool, 6> changed = {
+    back. process(),
+    left. process(),
+    right.process(),
+    up.   process(),
+    down. process(),
+    enter.process()
+  };
+  return std::any_of(changed.begin(), changed.end(), [](bool c){ return c; });
+}
+
 //-----------------------------------------------------------------------------
 
 void sound::beep()

cpp/ev3dev.h

@@ -1308,7 +1308,7 @@ class button
 
   // Check if the button state has changed,
   // call onclick function in case it has.
-  // Returns true if the state has changed, false otherwise.
+  // Returns true if the state has changed since the last call.
   bool process();
 
   static button back;
@@ -1318,6 +1318,10 @@ class button
   static button down;
   static button enter;
 
+  // Call process() for each of the EV3 buttons.
+  // Returns true if any of the states have changed since the last call.
+  static bool process_all();
+
 private:
   int _bit;
   bool _state = false;