Enable switching to DIRECT mode using the mouse.

This is about addressing the problem where selecting DIRECT with the mouse to switch input modes doesn't work.

In the PropertyHandler::ProcessPropertyActivate function, the processing for DIRECT mode is being skipped. This is because DIRECT=0 as defined in src/protocol/commands.proto#L51, so !entry->composition_mode is evaluated as true.
As a result, while the DIRECT menu item exists, it is not linked to the actual processing that should be executed for DIRECT.

src/unix/ibus/property_handler.cc

@@ -452,7 +452,7 @@ void PropertyHandler::ProcessPropertyActivate(IbusEngineWrapper *engine,
 
       const MozcEngineProperty *entry =
           prop.GetData<MozcEngineProperty>(kMozcEnginePropertyKey);
-      if (!entry || !entry->composition_mode) {
+      if (!entry) {
         continue;
       }
       SetCompositionMode(entry->composition_mode);