Expand sigil replacement for functions, enum values

This MR contains three commits, all of which modify gidocgen/mdext.py.

1. Create a SigilReplacement class which handles the replacement of sigil strings in the documentation, encapsulating both the pattern and replacement strings together into a single object for readability. All of the existing patterns and replacements are converted into SigilReplacement instances which are stored in a tuple, then applied in order programmatically. This is entirely a code-readability / quality-of-life change.

2. Adjust the SigilReplacement("Function") pattern so that functions followed by punctuation will be matched and processed. Previously, this would be matched:

   Call gtk_image_new_from_icon_name() to construct.

   But this would not:

   Call gtk_image_new_from_icon_name(), the constructor.

3. Add a SigilReplacement("Enum") to match enum values, which previously were not being replaced.

The last change can help avoid MarkDown parsing disasters like the following (from the live Gtk3 docs):

With the code in this MR, that becomes: