placeholder_t: Make char*s const

Similarly to https://github.com/i3/i3status/pull/412

include/libi3.h

@@ -543,9 +543,9 @@ int mkdirp(const char *path, mode_t mode);
 /** Helper structure for usage in format_placeholders(). */
 typedef struct placeholder_t {
     /* The placeholder to be replaced, e.g., "%title". */
-    char *name;
+    const char *name;
     /* The value this placeholder should be replaced with. */
-    char *value;
+    const char *value;
 } placeholder_t;
 
 /**

src/con.c

@@ -2318,11 +2318,11 @@ i3String *con_parse_title_format(Con *con) {
     char *formatted_str = format_placeholders(con->title_format, &placeholders[0], num);
     i3String *formatted = i3string_from_utf8(formatted_str);
     i3string_set_markup(formatted, pango_markup);
-    FREE(formatted_str);
 
-    for (size_t i = 0; i < num; i++) {
-        FREE(placeholders[i].value);
-    }
+    free(formatted_str);
+    free(title);
+    free(class);
+    free(instance);
 
     return formatted;
 }