tui: "backpressure": Drop messages to avoid flooding.

Closes #1234

multiqueue:
- Implement multiqueue_size()
- Rename MultiQueueItem.parent to MultiQueueItem.parent_item, to avoid confusion
  with MultiQueue.parent.

src/nvim/version.c

@@ -13,6 +13,7 @@
 #include "nvim/iconv.h"
 #include "nvim/version.h"
 #include "nvim/charset.h"
+#include "nvim/macros.h"
 #include "nvim/memline.h"
 #include "nvim/memory.h"
 #include "nvim/message.h"
@@ -22,9 +23,6 @@
 // version info generated by the build system
 #include "auto/versiondef.h"
 
-#define STR_(x) #x
-#define STR(x) STR_(x)
-
 // for ":version", ":intro", and "nvim --version"
 #ifndef NVIM_VERSION_MEDIUM
 #define NVIM_VERSION_MEDIUM STR(NVIM_VERSION_MAJOR) "." STR(NVIM_VERSION_MINOR)\