Parse the title_format and display the customized window title if a format was set.

The format string set with "title_format" can contain the placeholder "%title" which will be replaced with the actual window title.

By not overwriting window->name itself, we make sure that assignment matching still works as expected.

fixes #1723

include/libi3.h

@@ -243,6 +243,11 @@ bool i3string_is_markup(i3String *str);
  */
 void i3string_set_markup(i3String *str, bool is_markup);
 
+/**
+ * Escape pango markup characters in the given string.
+ */
+i3String *i3string_escape_markup(i3String *str);
+
 /**
  * Returns the number of glyphs in an i3String.
  *
@@ -381,6 +386,12 @@ xcb_char2b_t *convert_utf8_to_ucs2(char *input, size_t *real_strlen);
  */
 void set_font_colors(xcb_gcontext_t gc, uint32_t foreground, uint32_t background);
 
+/**
+ * Returns true if and only if the current font is a pango font.
+ *
+ */
+bool font_is_pango(void);
+
 /**
  * Draws text onto the specified X drawable (normally a pixmap) at the
  * specified coordinates (from the top left corner of the leftmost, uppermost