Fix incompatible type conversion errors

- A recent libxml2 changed its handler function to take a `const` pointer. It's
safe for us to have it, and them not, but the opposite way causes an implicit
cast warning.
- In relatively new GLib (many years now), `g_object_ref` casts its output to
match its input. This means we should not be casting to `G_OBJECT`, as that
is not the type it expects, and would translate to the output being `GObject`
instead of the original type.

This fixes the build in Fedora 40, which changes several incompatible pointer
conversion warnings into errors.

This patch is partially from the Fedora maintainer @yarda, with some corrections by me (to the `g_object_ref` portions.)

Testing Done:
Compiled in a Rawhide environment with this patch applied.

Bugs closed: PIDGIN-17850

Reviewed at https://reviews.imfreedom.org/r/2944/