feat(key_binder): bind key to a key sequence

Closes #301

To bind a key to a key sequence,
set 'send_sequence' attribute of a 'key_binder/bindings' entry.

Special named keys should be around with "{" and "}".
Notice that you have to add quotes when writing a filed with "{" or "}" in a
yaml file.

For example:
key_binder:
  bindings:
    - { when: composing, accept: a, send_sequence: aa }
    - { when: composing, accept: Right, send_sequence: "{Right}{Right}" }
    - { when: composing, accept: Left, send_sequence: "aa{Left}{Left}" }

src/rime/gear/key_binder.cc

@@ -48,7 +48,7 @@ static KeyBindingCondition translate_condition(const string& str) {
 
 struct KeyBinding {
   KeyBindingCondition whence;
-  KeyEvent target;
+  KeySequence target;
   function<void (Engine* engine)> action;
 
   bool operator< (const KeyBinding& o) const {
@@ -118,11 +118,20 @@ void KeyBindings::LoadBindings(const an<ConfigList>& bindings) {
       continue;
     }
     if (auto target = map->GetValue("send")) {
-      if (!binding.target.Parse(target->str())) {
+      KeyEvent key;
+      if (key.Parse(target->str())) {
+        binding.target.push_back(std::move(key));
+      } else {
         LOG(WARNING) << "invalid key binding #" << i << ".";
         continue;
       }
     }
+    else if (auto target = map->GetValue("send_sequence")) {
+      if (!binding.target.Parse(target->str())) {
+        LOG(WARNING) << "invalid key sequence #" << i << ".";
+        continue;
+      }
+    }
     else if (auto option = map->GetValue("toggle")) {
       binding.action = std::bind(&toggle_option, _1, option->str());
     }
@@ -203,7 +212,9 @@ void KeyBinder::PerformKeyBinding(const KeyBinding& binding) {
   }
   else {
     redirecting_ = true;
-    engine_->ProcessKey(binding.target);
+    for (const KeyEvent& key_event : binding.target) {
+      engine_->ProcessKey(key_event);
+    }
     redirecting_ = false;
   }
 }