feat(key_binder): add when: predicting condition (rime#751)

the new condition `when: predicting` allows paging key binding to comma,
period keys to be disabled when the active menu has tag `prediction`.

it's used with the `predictor` component in the librime-predict plugin.
see updates in data/minimal/default.yaml for example configuration.

src/rime/gear/key_binder.cc

@@ -22,16 +22,18 @@ namespace rime {
 
 enum KeyBindingCondition {
   kNever,
-  kWhenPaging,     // user has changed page
-  kWhenHasMenu,    // at least one candidate
-  kWhenComposing,  // input string is not empty
+  kWhenPredicting,  // showing prediction candidates
+  kWhenPaging,      // user has changed page
+  kWhenHasMenu,     // at least one candidate
+  kWhenComposing,   // input string is not empty
   kAlways,
 };
 
 static struct KeyBindingConditionDef {
   KeyBindingCondition condition;
   const char* name;
-} condition_definitions[] = {{kWhenPaging, "paging"},
+} condition_definitions[] = {{kWhenPredicting, "predicting"},
+                             {kWhenPaging, "paging"},
                              {kWhenHasMenu, "has_menu"},
                              {kWhenComposing, "composing"},
                              {kAlways, "always"},
@@ -242,8 +244,14 @@ KeyBindingConditions::KeyBindingConditions(Context* ctx) {
   }
 
   Composition& comp = ctx->composition();
-  if (!comp.empty() && comp.back().HasTag("paging")) {
-    insert(kWhenPaging);
+  if (!comp.empty()) {
+    const Segment& last_seg = comp.back();
+    if (last_seg.HasTag("paging")) {
+      insert(kWhenPaging);
+    }
+    if (last_seg.HasTag("prediction")) {
+      insert(kWhenPredicting);
+    }
   }
 }