Adds oneshot layer and oneshot tap toggling (#308)

This commit is mostly a cherry-pick from `ahtn` at
https://github.com/tmk/tmk_keyboard/pull/255.

These are the changes:

* Adds ACTION_LAYER_ONESHOT
* Adds ONESHOT_TAP_TOGGLE
* Mentions sticky keys in the docs on oneshot.

1 files changed, 4 insertions, 1 deletions

diff --git a/tmk_core/common/action_code.h b/tmk_core/common/action_code.h
index 2b0b0b077..ca729aaec 100644
--- a/tmk_core/common/action_code.h
+++ b/tmk_core/common/action_code.h
@@ -76,7 +76,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
  * 101E|LLLL|1111 0001   On/Off                 (0xF1)   [NOT TAP]
  * 101E|LLLL|1111 0010   Off/On                 (0xF2)   [NOT TAP]
  * 101E|LLLL|1111 0011   Set/Clear              (0xF3)   [NOT TAP]
- * 101E|LLLL|1111 xxxx   Reserved               (0xF4-FF)
+ * 101E|LLLL|1111 0100   One Shot Layer         (0xF4)   [TAP]
+ * 101E|LLLL|1111 xxxx   Reserved               (0xF5-FF)
  *   ELLLL: layer 0-31(E: extra bit for layer 16-31)
  *
  *
@@ -250,6 +251,7 @@ enum layer_pram_tap_op {
     OP_ON_OFF,
     OP_OFF_ON,
     OP_SET_CLEAR,
+    OP_ONESHOT,
 };
 #define ACTION_LAYER_BITOP(op, part, bits, on)      (ACT_LAYER<<12 | (op)<<10 | (on)<<8 | (part)<<5 | ((bits)&0x1f))
 #define ACTION_LAYER_TAP(layer, key)                (ACT_LAYER_TAP<<12 | (layer)<<8 | (key))
@@ -266,6 +268,7 @@ enum layer_pram_tap_op {
 #define ACTION_LAYER_ON_OFF(layer)                  ACTION_LAYER_TAP((layer), OP_ON_OFF)
 #define ACTION_LAYER_OFF_ON(layer)                  ACTION_LAYER_TAP((layer), OP_OFF_ON)
 #define ACTION_LAYER_SET_CLEAR(layer)               ACTION_LAYER_TAP((layer), OP_SET_CLEAR)
+#define ACTION_LAYER_ONESHOT(layer)                 ACTION_LAYER_TAP((layer), OP_ONESHOT)
 #define ACTION_LAYER_MODS(layer, mods)              ACTION_LAYER_TAP((layer), 0xe0 | ((mods)&0x0f))
 /* With Tapping */
 #define ACTION_LAYER_TAP_KEY(layer, key)            ACTION_LAYER_TAP((layer), (key))