Modernize minidialog

* Rename `LAST_PROPERTY` to `PROP_LAST`.
* Use GObject macros for declaration.
* Use GObject macros for definition.
* Move all internal property handling together.
* Move class init/finalize together with the rest of GObject implementation.
* Move callbacks together.
* Move helpers together (or inline).
* Add `g_return_if_fail(PIDGIN_IS_MINI_DIALOG(...))` on public API.

Testing Done:
Compile and connect an account with some buddy requests.

Reviewed at https://reviews.imfreedom.org/r/787/