Add ShiftBlocker example plugin

This mini plugin is an example of how to suppress a held `shift` key from a
Macros sequence.  In this case, it's to enable a macro that types an accented
character using a dead key (where the dead key must be entered without the
`shift` modifier held).

Signed-off-by: Michael Richters <gedankenexperimenter@gmail.com>

examples/Features/ShiftBlocker/ShiftBlocker.ino

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+/* -*- mode: c++ -*-
+ * ShiftBlocker -- A Kaleidoscope Example
+ * Copyright (C) 2016-2022  Keyboard.io, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify it under
+ * the terms of the GNU General Public License as published by the Free Software
+ * Foundation, version 3.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "Kaleidoscope.h"
+#include "Kaleidoscope-Macros.h"
+
+/* *INDENT-OFF* */
+KEYMAPS(
+  [0] = KEYMAP_STACKED
+  (
+   Key_NoKey,    Key_1, Key_2, Key_3, Key_4, Key_5, Key_NoKey,
+   Key_Backtick, Key_Q, Key_W, Key_E, Key_R, Key_T, Key_Tab,
+   Key_PageUp,   Key_A, Key_S, Key_D, Key_F, Key_G,
+   Key_PageDown,   Key_Z, Key_X, Key_C, Key_V, Key_B, Key_Escape,
+
+   Key_LeftControl, Key_Backspace, Key_LeftGui, Key_LeftShift,
+   M(0),
+
+   Key_skip,  Key_6, Key_7, Key_8,     Key_9,      Key_0,         Key_skip,
+   Key_Enter, Key_Y, Key_U, Key_I,     Key_O,      Key_P,         Key_Equals,
+              Key_H, Key_J, Key_K,     Key_L,      Key_Semicolon, Key_Quote,
+   Key_skip,  Key_N, Key_M, Key_Comma, Key_Period, Key_Slash,     Key_Minus,
+
+   Key_RightShift, Key_RightAlt, Key_Spacebar, Key_RightControl,
+   M(0)
+   ),
+)
+/* *INDENT-ON* */
+
+namespace kaleidoscope {
+namespace plugin {
+
+// When activated, this plugin will suppress any `Shift` key (including modifier
+// combos with `Shift`) before it's added to the HID report.
+class ShiftBlocker : public Plugin {
+
+ public:
+  EventHandlerResult onAddToReport(Key key) {
+    if (active_ && key.isKeyboardShift())
+      return EventHandlerResult::ABORT;
+    return EventHandlerResult::OK;
+  }
+
+  void enable() {
+    active_ = true;
+  }
+  void disable() {
+    active_ = false;
+  }
+
+ private:
+  bool active_{false};
+
+};
+
+} // namespace plugin
+} // namespace kaleidoscope
+
+kaleidoscope::plugin::ShiftBlocker ShiftBlocker;
+
+const macro_t *macroAction(uint8_t macro_id, KeyEvent &event) {
+  if (keyToggledOn(event.state)) {
+    switch (macro_id) {
+    case 0:
+      // First, enable ShiftBlocker to suppress any held `Shift` key(s).
+      ShiftBlocker.enable();
+      // Tap `AltGr` + `7` to activate the grave accent dead key.
+      Macros.tap(RALT(Key_7));
+      // Disable ShiftBlocker so it won't affect the `E` event.
+      ShiftBlocker.disable();
+      // Change the Macros key into a plain `E` key before its press event is
+      // processed.
+      event.key = Key_E;
+      break;
+    }
+  }
+  return MACRO_NONE;
+}
+
+KALEIDOSCOPE_INIT_PLUGINS(Macros,
+                          ShiftBlocker);
+
+void setup() {
+  Kaleidoscope.setup();
+  // Uncomment to manually set the OS, as Kaleidoscope will not autodetect it.
+  // (Possible values are in HostOS.h.)
+  // HostOS.os(kaleidoscope::hostos::LINUX);
+}
+
+void loop() {
+  Kaleidoscope.loop();
+}

examples/Features/ShiftBlocker/sketch.json

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+{
+  "cpu": {
+    "fqbn": "keyboardio:avr:model01",
+    "port": ""
+  }
+}