ui: Add abstract_ui termcap and split UI layer

This is how Nvim behaves when the "abstract_ui" termcap is activated:

- No data is written/read to stdout/stdin by default.
- Instead of sending data to stdout, ui_write will parse the termcap codes
  and invoke dispatch functions in the ui.c module.
- The dispatch functions will forward the calls to all attached UI
  instances(each UI instance is an implementation of the UI layer and is
  registered with ui_attach).
- Like with the "builtin_gui" termcap, "abstract_ui" does not contain any key
  sequences. Instead, vim key strings(<cr>, <esc>, etc) are parsed directly by
  input_enqueue and the translated strings are pushed to the input buffer.

With this new input model, its not possible to send mouse events yet. Thats
because mouse sequence parsing happens in term.c/check_termcodes which must
return early when "abstract_ui" is activated.

src/nvim/os/input.c

@@ -46,7 +46,7 @@ void input_init(void)
 {
   input_buffer = rbuffer_new(INPUT_BUFFER_SIZE + MAX_KEY_CODE_LEN);
 
-  if (embedded_mode) {
+  if (abstract_ui) {
     return;
   }
 
@@ -57,7 +57,7 @@ void input_init(void)
 
 void input_teardown(void)
 {
-  if (embedded_mode) {
+  if (abstract_ui) {
     return;
   }
 
@@ -67,7 +67,7 @@ void input_teardown(void)
 // Listen for input
 void input_start(void)
 {
-  if (embedded_mode) {
+  if (abstract_ui) {
     return;
   }
 
@@ -77,7 +77,7 @@ void input_start(void)
 // Stop listening for input
 void input_stop(void)
 {
-  if (embedded_mode) {
+  if (abstract_ui) {
     return;
   }
 
@@ -180,7 +180,23 @@ void input_buffer_restore(String str)
 
 size_t input_enqueue(String keys)
 {
-  size_t rv = rbuffer_write(input_buffer, keys.data, keys.size);
+  char *ptr = keys.data, *end = ptr + keys.size;
+
+  while (rbuffer_available(input_buffer) >= 6 && ptr < end) {
+    int new_size = trans_special((char_u **)&ptr,
+        (char_u *)rbuffer_write_ptr(input_buffer),
+        false);
+    if (!new_size) {
+      // copy the character unmodified
+      *rbuffer_write_ptr(input_buffer) = *ptr++;
+      new_size = 1;
+    }
+    // TODO(tarruda): Don't produce past unclosed '<' characters, except if
+    // there's a lot of characters after the '<'
+    rbuffer_produced(input_buffer, (size_t)new_size);
+  }
+
+  size_t rv = (size_t)(ptr - keys.data);
   process_interrupts();
   return rv;
 }
@@ -255,7 +271,7 @@ static void read_cb(RStream *rstream, void *data, bool at_eof)
 
 static void convert_input(void)
 {
-  if (embedded_mode || !rbuffer_available(input_buffer)) {
+  if (abstract_ui || !rbuffer_available(input_buffer)) {
     // No input buffer space
     return;
   }
@@ -335,7 +351,7 @@ static bool input_ready(void)
   return typebuf_was_filled ||                 // API call filled typeahead
          rbuffer_pending(input_buffer) > 0 ||  // Stdin input
          event_has_deferred() ||               // Events must be processed
-         (!embedded_mode && eof);              // Stdin closed
+         (!abstract_ui && eof);                // Stdin closed
 }
 
 // Exit because of an input read error.