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

libi3/string.c

@@ -13,6 +13,10 @@
 #include <stdlib.h>
 #include <string.h>
 
+#if PANGO_SUPPORT
+#include <glib.h>
+#endif
+
 #include "libi3.h"
 
 struct _i3String {
@@ -185,6 +189,18 @@ void i3string_set_markup(i3String *str, bool is_markup) {
     str->is_markup = is_markup;
 }
 
+/*
+ * Escape pango markup characters in the given string.
+ */
+i3String *i3string_escape_markup(i3String *str) {
+#if PANGO_SUPPORT
+    const char *text = i3string_as_utf8(str);
+    return i3string_from_utf8(g_markup_escape_text(text, -1));
+#else
+    return str;
+#endif
+}
+
 /*
  * Returns the number of glyphs in an i3String.
  *