string: Add find and replace function

This adds g_string_replace(), a function that replaces instances of one string
with another in a GString. It allows the caller to specify the maximum number
of replacements to perform, and returns the number of replacements performed
to the caller.

Fixes: #225

docs/reference/glib/glib-sections.txt

@@ -2861,6 +2861,7 @@ g_string_insert_unichar
 g_string_insert_len
 g_string_overwrite
 g_string_overwrite_len
+g_string_replace
 g_string_erase
 g_string_truncate
 g_string_set_size