Add style-escape methods to ui_file

This adds emit_style_escape and reset_style methods to ui_file.  These
aren't used yet, but they will be once the pager is converted to be a
ui_file subclass.

gdb/ui-file.c

@@ -73,6 +73,30 @@ ui_file::vprintf (const char *format, va_list args)
 
 /* See ui-file.h.  */
 
+void
+ui_file::emit_style_escape (const ui_file_style &style)
+{
+  if (can_emit_style_escape () && style != m_applied_style)
+    {
+      m_applied_style = style;
+      this->puts (style.to_ansi ().c_str ());
+    }
+}
+
+/* See ui-file.h.  */
+
+void
+ui_file::reset_style ()
+{
+  if (can_emit_style_escape ())
+    {
+      m_applied_style = ui_file_style ();
+      this->puts (m_applied_style.to_ansi ().c_str ());
+    }
+}
+
+/* See ui-file.h.  */
+
 void
 ui_file::printchar (int c, int quoter, bool async_safe)
 {

gdb/ui-file.h

@@ -124,6 +124,12 @@ public:
      used to force out output from the wrap_buffer.  */
   void wrap_here (int indent);
 
+  /* Emit an ANSI style escape for STYLE.  */
+  virtual void emit_style_escape (const ui_file_style &style);
+
+  /* Rest the current output style to the empty style.  */
+  virtual void reset_style ();
+
   /* Print STR, bypassing any paging that might be done by this
      ui_file.  Note that nearly no code should call this -- it's
      intended for use by printf_filtered, but nothing else.  */
@@ -132,6 +138,11 @@ public:
     this->puts (str);
   }
 
+protected:
+
+  /* The currently applied style.  */
+  ui_file_style m_applied_style;
+
 private:
 
   /* Helper function for putstr and putstrn.  Print the character C on
@@ -376,6 +387,14 @@ public:
      sequences.  */
   void write (const char *buf, long length_buf) override;
   void puts (const char *linebuffer) override;
+
+  void emit_style_escape (const ui_file_style &style) override
+  {
+  }
+
+  void reset_style () override
+  {
+  }
 };
 
 /* A ui_file that optionally puts a timestamp at the start of each