Add a command-mode mapping for ':' instead of handling it specially

src/core/Command.h

@@ -48,6 +48,7 @@ public:
         SCROLL_WINDOW_DOWN_HALF_SCREEN,
         SCROLL_WINDOW_UP_WHOLE_SCREEN,
         SCROLL_WINDOW_DOWN_WHOLE_SCREEN,
+        ENTER_COMMAND_LINE_PROMPT,
 
         COMMAND_COUNT,
     };

src/core/DefaultCommandMap.cc

@@ -49,6 +49,7 @@ void DefaultCommandMap::build()
     dcm->add("j", Command::GO_DOWN, nullptr, false);
     dcm->add("k", Command::GO_UP, nullptr, false);
     dcm->add("l", Command::GO_RIGHT, nullptr, false);
+    dcm->add(":", Command::ENTER_COMMAND_LINE_PROMPT, nullptr, false);
 
     dcm->add("x", Command::DELETE_CHAR, nullptr, false);
     dcm->add("X", Command::DELETE_CHAR_BACK, nullptr, false);