text: Add delete_surrounding_text to protocol

Add delete_surrounding_text event in the text_model interface and the
request in the input_method_context interface. Implement it in the
example editor client and in the example keyboard so that the backspace
key works with it.

Signed-off-by: Jan Arne Petersen <jpetersen@openismus.com>

tests/test-text-client.c

@@ -59,6 +59,14 @@ text_model_preedit_string(void *data,
 {
 }
 
+static void
+text_model_delete_surrounding_text(void *data,
+				   struct text_model *text_model,
+				   int32_t index,
+				   uint32_t length)
+{
+}
+
 static void
 text_model_preedit_styling(void *data,
 			   struct text_model *text_model)
@@ -112,6 +120,7 @@ text_model_deactivated(void *data,
 static const struct text_model_listener text_model_listener = {
 	text_model_commit_string,
 	text_model_preedit_string,
+	text_model_delete_surrounding_text,
 	text_model_preedit_styling,
 	text_model_key,
 	text_model_selection_replacement,